bluuebunny/crossref_metadata_2025 huggingface dataset:
Separation of Uranium and Zirconium in Alkali Chloride Melts Using Liquid Metal Cathodes
Authors: Volkovich Vladimir A. , Maltsev Dmitry , Soldatova Maria N. , Ryzhov Alexander A. , Ivanov Alexander B.
Published: December 2020

Alkali chloride melts are considered as prospective working media for non-aqueous pyrochemical reprocessing of spent nuclear fuels (SNFs). Separation of fissile materials from fission products in pyrochemical reprocessing can be achieved electrochemically and amongst all the fission products zirconium has the closest electrochemical properties to uranium. Uranium and plutonium fission produces several zirconium isotopes (from Zr-91 to Zr-97) and depending on the reactor neutron spectrum, nuclear fuel type, burnup and cooling time SNF arriving for reprocessing can contain ca. 5–13 kg of zirconium (as the fission product excluding cladding) per ton. Electrochemical separation of uranium and zirconium in fused salts is a challenging task.

The present work was devoted to studying the electrochemical behavior of zirconium and uranium in 3LiCl–2KCl based melts using cyclic voltammetry and cathodic polarization. The experiments were performed in LiCl–KCl–ZrCl4, LiCl–KCl–UCl4, LiCl–KCl–UCl3 and LiCl–KCl–ZrCl4–UCl4 melts.

On a solid tungsten electrode zirconium(IV) ions were reduced to Zr(0) in two stages and the metal deposition potentials were between –2.17 and –2.07 V (at 532–637 oC) vs. Cl–/Cl2 couple. Changing the solid electrode to liquid zinc, gallium or gallium–zinc eutectic alloy (3.64 wt. % Zn) resulted in significant shift of zirconium deposition potential in the positive direction. Examples of the polarization curves are shown in the Fig. Polarization measurements performed in LiCl–KCl–ZrCl4–UCl4 melt on the Ga–Zn electrode showed that reduction of U(IV) to U(III) and deposition of zirconium occurred at very similar potentials and deposition potential of uranium was significantly more negative. Separation factor for uranium/zirconium couple was also determined.

Fig. Polarization of Ga–Zn eutectic alloy cathode in LiCl–KCl–UCl4 (524 oC) and LiCl–KCl–ZrCl4 (550 oC) melts.

Figure 1
Electrochemical Behavior of Silver Halogenocomplexes in an Amide-Type Ionic Liquid
Authors: Serizawa Nobuyuki , Kuwahara Suguru , Katayama Yasushi
Published: December 2020

Silver halides, AgX (X = Cl–, Br–, and I–), are known to be typical poorly soluble salts in aqueous solution. AgX is also almost insoluble in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonly)amide (BMPTFSA). However, AgX has been known to dissolve in the media in the presence of X– by forming halogenocomplexes, [AgX
n
](n–1)–. Dissolution of AgCl has been reported in basic chloroaluminate ionic liquids[1] and deep eutectic solvents containing Cl–. In these reports, however, electrochemical behavior of [AgX
n
](n–1)– has not been studied in detail. We have already reported that some metal chlorides are soluble in BMPTFSA in the presence of Cl– and that electrodeposition of the metals is possible[2,3]. In the present study, dissolution of AgX has been investigated in BMPTFSA with addition of BMPX. Furthermore, electrodeposition of Ag and electrochemical preparation of Ag nanoparticles have been examined in the Lewis basic ionic liquids.

AgX was found to dissolve in BMPTFSA containing 0.5 M BMPX. The dissolved species was determined to be [AgX3]2– by potentiometric technique. The deposition and dissolution of Ag were observed in cyclic voltammetry. The redox potential of [AgX3]2–/Ag was more negative than that of [Ag(TFSA)3]–/Ag and dependent on the kind of halide. The diffusion coefficient of [AgX3]2– was determined by chlonoamperometry. The initial stage of electrodeposition of Ag on a glassy carbon electrode was regarded as progressive. Potentiostatic cathodic reduction of [AgX3]2– resulted in deposition of Ag. The morphology of Ag deposits depended on the reduction potential. Formation of Ag nanoparticles dispersed in the ionic liquid was confirmed after potentiostatic cathodic reduction at a negative potential using transmission electron microscopy[2].

References

[1] T. M. Laher and C. L. Hussey, Inorg. Chem., 22, 1279 (1983).

[2] Y. Katayama, Y. Oshino, N. Ichihashi, N. Tachikawa, K. Yoshii, and K. Toshima, Electrochim. Acta, 183, 37 (2015).

[3] M. Sano, N. Tachikawa, K. Yoshii, N. Serizawa, and Y. Katayama, Electrochemistry, 86, 260 (2018).
Electrochemical Investigation of (De-)Lithiation Behaviors of Al Anodes and the Solubility Range of β-Lial Phase at Room Temperature
Authors: Zheng Tianye , Kramer Dominik , Tahmasebi Mohammad H. , Geßwein Holger , Mönig Reiner , Boles Steven T.
Published: December 2020

Alloy anodes have been extensively studied over past decades to replace the graphite of lithium-ion batteries (LIBs). Most efforts are made towards those anode materials with high specific capacities such as silicon and germanium. To minimize the negative impacts of the volume expansions during Li incorporation, protective coatings or specific micro- and nanostructures have been suggested, but a commercialization on a larger scale was impeded by such preparation costly procedures. Therefore, the optimal alloy anode candidate should balance the cost, capacity and manufacturing complexity rather than solely maximize the specific capacity. Aluminum, an underappreciated anode material, possesses intrinsic properties that fulfill the criteria mentioned above. The origin of the main capacity of aluminum anode is upon the formation of the β phase (LiAl; ca. 1Ah/g). However, the (de-)alloying processes of Al remain poorly understood, especially at room temperature under which LIBs usually operate.

In this study, we investigate the (de-)lithiation that occurs within the solubility range of the β phase Although this Li solubility is highlighted by multiple versions of the Li-Al phase diagrams, the solubility range at low temperatures was not specified yet due to the experimental conditions of previous phase diagram studies (> 400 °C). Various electrochemical analytic tools and methods are used to shed light on the Li solubility within the β phase. The Li solubility range of the β phase is determined to be roughly 5 at% by a potentiostatic charge counting experiment at room temperature. Moreover, the cyclic voltammetry of partially lithiated Al foils shows that the β phase can be (de-)saturated without propagating the phase front towards the α phase. If galvanostatic charge and discharge is smartly controlled by taking into consideration of the solubility range, the cycling life of β-LiAl anode can be significantly improved. Not only does this piece of work provide fundamental data for the β-LiAl phase at room temperature that complements the existing phase diagrams, but also implies that aluminum foil holds great potential as an anode material for the next generation of lithium-based batteries.
Accelerating Water Dissociation in Bipolar Membranes and Electrocatalysis
Authors: Boettcher Shannon W.
Published: December 2020

Catalyzing water dissociation (WD), H2O -> H+ + OH-, is practically important for fabricating bipolar membranes (BPMs) that couple different pH environments into a single electrochemical device and for accelerating (electro)catalytic reactions that consume water. We use a BPM electrolyzer for quantitative measurement of WD kinetics in the absence of soluble electrolyte. We discover that the rate of WD on metal-oxide nanoparticle surfaces is affected by the local pH, consistent with a proton-transfer mechanism. We combine WD catalysts efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer and yield WD overpotentials < 10 mV across the BPM at 20 mA cm-2. We demonstrate pure-water BPM electrolyzers that maintain a gradient of ~ 14 pH units between the basic anode and acidic cathode that operate continuously at 500 mA cm-2 with a total electrolysis voltage of ~ 2.2 V. We further investigate the WD activity of metal electrocatalysts in the BPM junction and find a direct correlation with their electrocatalytic activity for the hydrogen-evolution reaction (HER) in alkaline media. This result is direct evidence of the rate-limiting role of WD in alkaline HER and guides advanced (electro)catalyst design for (electrochemical) reactions where water is a reactant.
Electrocatalytic Conversion of CO2 over Bimetallic Electrocatalysts - Relationship between the Selectivity and Surface Structure of Catalysts
Authors: Gunji Takao
Published: December 2020

Conversion reaction of CO2 by electrolysis on heterogeneous catalysts have been attractive attention due to not only decreasing the amount of CO2 of greenhouse gas but also mitigate the exhaust of energy resources. However, since CO2 is the most stable compounds in the carbon based owning to its C=O bond in CO2 molecular, it is significantly difficult to convert CO2 into other compounds such as hydrocarbons as well as HCOOH, and CO.

So far, several efforts have been made to improve both electrocatalytic activity and selectivity towards CO2 reduction reaction. It is well-known that novel metals such as Pd, Au and Ag possesses a CO-selectivity, and non-precious metals such as Pb, Bi, and Sn exhibit a formate selectivity. In particular, Cu is attracting much attention as the electrocatalyst for electrochemical CO2 reduction reaction because it is proved as a unique electrocatalyst for CO2 reduction; metallic Cu possesses organic compounds (i.e., methane, ethane, ethylene, and/or methanol) selectivity. However, formation of organic compounds from the CO2 reduction reaction requires a high overpotential when Cu is used as electrocatalyst. In order to prepare a novel electrocatalyst which shows a high selectivity for organics production with low overpotentials, we attempt to prepare Pd based atomically ordered intermetallic compounds.

Today, I present recent two topics; (i) investigating electrocatalytic selectivity of Pd-based alloys toward the CO2 reduction reaction, depending on second metals, and (2) how can be prepared the Pd-based intermetallic compounds which exhibit the organic selectivity.
Understanding the Catalytic Site in RuOx-TiOx Electrodes for Chlorine Evolution Reaction
Authors: Dong Heng , Hoffmann Michael R.
Published: December 2020

As late as in 2017, there were still at least one third of world population suffering from water sanitation problems. Electrochemical oxidation (EO) technology, which relies on anodic generation of reactive chlorine and oxygen species as oxidants and disinfectants, has been shown feasible for human wastewater onsite treatment and recycling. Pivotal to the EO system is anodic chlorine evolution reaction (CER). Traditionally, oxides of platinum group metals (PGMs) have shown excellent catalytic activity for CER, with RuO2 identified as the golden standard. However, fast acid dissolution and prohibitive price prevents its real application. On the other hand, RuOx-TiOx binary electrodes (denoted as RTO electrodes) have demonstrated both enhanced activity and stability compared to pure RuO2, and it has been used in chloro-alkali industry for centuries as one of the dimensionally stable anodes (DSA). Previous research has ascribed the exceptional activity of RTO electrodes to Ti-Ru polarizations pairs (denoted as RuOx
𝛿
+-TiOx
𝛿
-). Nevertheless, the actual catalytic site (RuOx
𝛿
+ or TiOx
𝛿
-), as well as the specific mechanism, has remained elusive.

In this work, ultrathin sandwich electrodes were fabricated by sequentially depositing nanometer-thick uniform crystalline TiO2 and RuO2 films onto HF-treated atomically flat Si wafers to study the active sites in RTO electrodes. The electrodes were characterized with scanning electron microscope (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM), while the CER catalytic activity was probed by chronopotentiometry experiments in 5 M NaCl solution. The structure of the sandwich electrodes was varied to investigate the activity of both components, and layer thickness was varied to quantify the effective range of Ti-Ru polarizations. Density functional theory (DFT) computation was used to help understand the catalytic mechanism.

In order to promote further application of EO systems in developing countries, electrodes at lower price are essential. Not only does this work shed light on the catalytic mechanism of RTO electrodes, it also provides important guidance for design of low-PGM or PGM-free electrodes towards high-efficiency CER catalysis and wastewater treatment.
The Effect of Additives on the Performance and Failure Mechanisms of the Rechargeable, Low-Cost, Alkaline Zinc Electrode
Authors: D'Ambrose Michael Joseph , Turney Damon , Yadav Gautam Ganapati , Nyce Michael , Messinger Robert J. , Banerjee Sanjoy
Published: December 2020

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, non-flammable, earth abundant metal that is compatible with aqueous electrolytes. Zinc metal is also energy dense, exhibiting a high gravimetric discharge capacity of 820 mAh/g, a high volumetric discharge capacity of 5850 mAh/mL, and a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte. Rechargeable aqueous Zn alkaline batteries have thus recently been the subject of intense investigations for safe, low-cost, stationary electrochemical energy storage. Our research aims to achieve Zn depth-of-discharge (DOD) of 20% that is sustainable for 500 or more charge-discharge cycles, corresponding to an overall battery energy density of 200 Wh/L at the cell level.

However, the cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease strongly with increasing Zn DOD. For example, we recently demonstrated that metallic Zn electrodes cycled in the range of 1% to 16% Zn DOD had an exponential decrease in cycle life with Zn DOD, which was attributed mostly to the loss of active Zn material. Here, we study the effect of additives to increase the Zn DOD and cycle life of Zn electrodes. Zinc oxide (ZnO) was used as a starting material from which metallic Zn was formed in zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the ZnO electrodes for the tests done on Zn-Ni batteries and these electrodes were cycled in the range of 15% to 30% Zn DOD. The potential values of the Zn electrodes vs. Hg|HgO reference electrodes were recorded to identify failure due to Zn. At 15% Zn DOD, ZnO electrodes with certain additives or combinations of additives achieved an increased cycle life compared to Zn electrodes without additives. This highlights the need to perform electrochemical and spectroscopic studies to determine the effects of the additives on the mechanisms of the Zn electrode. These additives, with further understanding, could be implemented in low-cost, energy-dense, rechargeable Zn alkaline batteries for grid-scale energy storage.
Toward Predicting Trends in Propagating Failure and Its Mitigation for a Lithium-Ion Cell Stack
Authors: Hewson John , Kurzawski Andrew , Shurtz Randy C. , Torres-Castro Loraine , Lamb Joshua
Published: December 2020

Thermal runaway of lithium-ion batteries is a risk that is magnified when stacks of lithium-ion cells are used for large scale energy storage. When limits of propagation can be identified so that systems can be designed to prevent large scale cascading failure even if a failure does occur, these systems will be safer. This work addresses the prediction of cell-to-cell failure propagation and the propagation limits in lithium-ion cell stacks to better understand and identify safe designs.

A thermal-runaway model is presented based on recent developments in thermochemical source terms. It is noted that propagating failure is characterized by temperatures above which calorimetry data is available. Results show high temperature propagating failure predictions are too rapid unless an intra-particle diffusion limit is included, introducing a Damkohler number limiter into the rate expression. This new model form is evaluated against cell-to-cell failure propagation where the end cell of a stack is forced into thermal runaway through a nail-induced short circuit. Limits of propagation for this configuration are identified in terms of the energy density and in terms of the thermal contact between cells. Results show cell-to-cell propagation predictions are consistent with measurements over a range of cell states of charge and with the introduction of metal plates between cells to add system heat capacity representative of structural members. This consistency extends from scenarios where propagation occurs through scenarios where propagation is prevented.

The authors gratefully acknowledge funding from the Department of Energy, Office of Electricity's Energy Storage Program, under the direction of Dr. Imre Gyuk. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
(Invited) Battery Durability and Reliability Under Electric Utility Grid Operations
Authors: Dubarry Matthieu , Baure George
Published: December 2020

Battery Energy Storage Systems (BESSs) show promise to help renewable energy sources integration onto the grid. These systems are expected to last for a decade or more, but the actual battery degradation under different real-world conditions is still largely unknown.

In this work we analyzed different types of usage for batteries similar to deployed 1MW and 2MW BESS in the Hawaiian archipelago. These batteries are used for a mix of different applications such as voltage smoothing, curtailment, reserve and frequency regulation. With different usages come different battery degradation. A new protocol encompassing all those applications was designed to be able to test the cells and assess their durability in the field. We investigated the impact of frequency regulation duty cycles with different SOC ranges, depth of discharge and temperatures. We also investigated cells with varying duty cycles and temperatures to decipher whether the degradation can be predicted from a laboratory experiment.

Based on the BESS representative usage profiles, we designed cycle-aging and calendar-aging experiments to test the degradation in a controlled fashion. The frequency regulation duty cycle was extracted from a 3 years usage analysis of a previous generation lithium titanate BESS in operation in Hawai'i since 2012. Laboratory testing was used to investigate the degradation in detail to improve remaining useful life predictions. The degradation mechanisms, and their path dependency, were analyzed using incremental capacity analysis and mechanistic modeling. It was found that despite the fact that most of the cells did less than 0.5% capacity after more than 400 days of testing, they actually degraded significantly. Cell temperature had the strongest impact on the loss of lithium inventory whereas duty cycle had a big impact on the loss of positive electrode material. Interestingly, the impact of SOC in the calendar aging experiments, was found to be counterintuitive as batteries lost capacity faster at low SOCs than at high. Based on our results we established a 20 year forecast of the BESS degradation under different usages.
Evaluation of Ageing and Degradation for Lithium-Ion Batteries By Using Electrochemical Harmonic Analysis (EHA)
Authors: Gulsoy Begum , Atalay Selcuk
Published: December 2020

Lithium-ion cells show a significant degradation in performance with time (1). This occurs as a result of physical and mechanical changes over time that impacts the electrochemical properties and operating parameters throughout the battery lifetime. These can affect the battery performance in terms of a rise in battery impedance and change in the potential difference that are known as a result of nonlinear electrochemical processes of batteries (2, 3). Therefore, the method of observing nonlinear behaviours have been outstanding to understanding and optimising battery performance (1, 3).

In this research, the influences of battery ageing and degradation on harmonic distortion in lithium-ion batteries are investigated to understand nonlinear behaviours of batteries by using Electrochemical Harmonic Analysis (EHA). The study analysed the effects of wide range of temperatures, C-rates and state of charge on battery ageing and degradation. For this purpose, aged and pristine cells are tested in order to analysis harmonic distortion at a specific frequency range between 100mHz and 10kHz.

The comparison of harmonic distortion at the same frequencies for aged and pristine cells illustrates that there is a relationship between ageing phenomena and harmonic distortion. The level of distortion increases proportionally according to the effects of ageing on the battery over time. The distortion was mostly seen in lower frequencies, which were in the range of 100mHz to 10Hz. This distortion proceeded directly from the diffusion process of lithium-ion cells. The findings indicate that lower levels of state of charge and higher temperatures have a decrease in total harmonic distortion (THD).

The principal conclusion of this study is that the difference in the behaviour of aged and pristine cells clearly indicates that EHA technique can be used to identify the degree of ageing and degradation on lithium-ion cells.

Keywords: Electrochemical Impedance Spectroscopy (EIS), Harmonic Analysis, Total Harmonic Distortion (THD), Distortion Integral (DI), Ageing, Degradation, Lithium-ion Battery, Temperature Effect, C-rates, State of Charge (SoC), State of Health (SoH).

N. Harting, R. Schenkendorf, N. Wolff and U. Krewer, Applied Sciences, 8, 821 (2018).

J. Vetter, P. Novák, M. R. Wagner, C. Veit, K.-C. Möller, J. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler and A. Hammouche, Journal of power sources, 147, 269 (2005).

M. Danzer, V. Liebau and F. Maglia, in Advances in Battery Technologies for Electric Vehicles, p. 359, Elsevier (2015).
The Roll-to-Roll Deposition of 2D Nanoflake Films of Semiconducting Tmds for Large Area Solar-to-Chemical Energy Conversion
Authors: Wells Rebekah , Johnson Hannah , Lhermitte Charles Roger , Kinge Sachin , Sivula Kevin
Published: December 2020

The exceptional semiconducting properties of exfoliated transition metal dichalcogenide (TMDs) nanosheets have long inspired their use in photoelectrochemical applications; in particular, photoelectrodes for solar-to-chemical energy conversion. Indeed, past demonstrations using non-exfoliated, bulk material gave solar-to-hydrogen efficiencies greater than 7%.1 In taking advantage of the exfoliable nature of these materials, nano-TMDs not only provide a new range of tunable optoelectronic properties, but also create a direct path toward ultrathin, flexible devices. However, a major challenge in commercializing TMD-based devices is their large-scale production.2 Our group has recently addressed this issue by developing methods for solution-based fabrication of nanoflake TMD thin films for large area solar energy conversion applications.3 In this presentation we highlight our results employing WSe2 for solar water splitting and report the further advancement of our preparation methods toward a continuous roll-to-roll deposition of TMD-based thin films from nanoflake dispersions using a liquid-liquid interfacial self-assembly technique.3,4 We demonstrate reproducible printing of 100mm wide TMD flake films on plastic substrates at 10 mm s-1 with nanoflake loadings of 35 mg m-2.4 Importantly we show that these large-scale films of semiconducting TMDs are robust and capable of solar-to-chemical energy conversion comparable to that of their batch-scale counterparts.4 In addition, we address the effects of processing conditions on TMD flake quality and optoelectronic device performance. We find that the type of exfoliation and related parameters have a significant impact on the defect density within the flakes, which effects the harvesting of photogenerated charge carriers. We observe this for MoS2 and WSe2-based devices, suggesting that this is a fundamental limitation of liquid exfoliated TMDs. To overcome this limitation we present distinct defect mitigation strategies that can drastically improve the solar energy conversion ability of TMD nanoflake films.5 These techniques lead to state-of -the-art performance of solution-processed WSe2 nanoflake films for photoelectrochemical H2 production with internal quantum efficiency of over 50% and 1 sun photocurrent density over 4 mA cm-2.5

References:

1. J. Am. Chem. Soc., 1, 223-231 (2013).

2. ACS
Energy Lett., 1, 315–322 (2016).

3. Nat. Commun., 6, 7596 (2015).

4. ACS Appl. Nano Mater., 2, 7705-7712 (2019).

5. Nano Lett., 18, 215–222 (2018).

Figure 1
Anodic TiO2 Nanotube Layers: Efficient Photocatalyst
Authors: Sopha Hanna , Motola Martin , Zazpe Raul , Ng Siowwoon , Macak Jan M.
Published: December 2020

The self-organized TiO2 nanotube layers have attracted considerable scientific and technological interest over the past 13 years motivated by their possible range of applications including photocatalysis, solar cells, hydrogen generation and biomedical uses [1,2]. The synthesis of these 1D TiO2 nanotube layers is carried out by a conventional electrochemical anodization of valve Ti metal sheet.

Among other TiO2 nanomaterials, these nanotube layers stand out due to their directionality, tunability of dimensions, ability to absorb significant amount of incident light and the possibility to utilize nanotube interiors and exteriors for decoration-coating of secondary materials.

One of the most significant applications of TiO2 nanomaterials is photocatalysis, which is very effective to decompose various pollutants (e.g. dyes) and kill bacteria [3,4].

The presentation will focus on the utilization of the nanotube layer for photocatalytic removal of various species in the liquid phase [5, 6] as well as gas phase [7]. We will also show that various secondary materials have a positive influence on photo-electrochemical properties of nanotube layers [8, 9]. Experimental details and some recent photocatalytic results will be presented and discussed.

References:

[1] J. M. Macak et al., Curr.
Opin. Solid State Mater.
Sci.,
2007, 1-2, 3.

[2] K. Lee et al., Chem. Rev., 2014, 114, 9385.

[3] K. Rajeshwar et al., J. Photochem. Photobiol. C: Photochem. Rev.
2008, 9, 171.

[4] Ch. Wei et al., Environ.Sci. Technol.
1994, 28, 934.

[5] J.M. Macak et al., Small, 2007, 3, 300.

[6] H. Sopha et al., Appl. Mater. Today,
2017, 9, 104.

[7] H. Sopha et al., Electrochem. Commun.,
2018, 97, 91.

[8] S. Ng et al., Adv. Mater. Interfaces
2017, 1701146.

[9] F. Dvorak et al., Appl. Mater. Today,
2019, 14, 1.
Heteronanostructured Photocatalysts with Epitaxial Junctions between the Components
Authors: Tada Hiroaki
Published: December 2020

Heteronanostructured (HNS) photocatalysts are the key material for various important solar-to-chemical transformations. In the semiconductor photoelectrodes, efficient charge separation can be achieved by the Schottky barrier. However, in the particulate HNS photocatalysts, the dimension of semiconductor is usually too small to accommodate the space charge layer. Even if the space charge layer can be formed at the initial stage, the band bending would become weak at the photostationary state due to the accumulation of the electrons in the conduction band. Thus, the major subject in the HNS photocatalysts is the enhancement of charge separation through the efficient interfacial charge transfer between the components for which the construction of the high-quality interface is crucial [1]. On the other hand, Au NP possesses strong and broad absorption in the visible region due to the localized surface plasmon resonance, and loading Au NPs on wide gap semiconductor photocatalysts such as TiO2, SnO2, and ZnO (Au/semiconductor) can induce the visible-light responsiveness and/or work as a catalyst for various reactions [1]. Importantly, the optical property and catalytic activity of Au NPs strongly depend on the size and shape.

This talk focusses on our recent works on the interfacial control in the HNSs at an atomic level, and the effects on the photocatalytic activity. Firstly, a novel nanoscale charge separation mechanism is presented in a HNS consisting of SnO2 nanorods and rutile TiO2 with a heteroepitaxial (HEPI) junction (SnO2-NR#TiO2, # denotes HEPI junction). SnO2-NR#TiO2 shows photocatalytic activity for selective oxidation of ethanol to acetaldehyde much higher than the physical mixture of SnO2 and TiO2 [2]. Secondly, the SnO2-NR#TiO2 charge separator was further modified with Au NPs (Au/SnO2-NR#TiO2) to impart visible-light activity. The Au/SnO2-NR#TiO2 plasmonic photocatalyst exhibits a high level of visible-light activity for the synthesis of H2O2 by two-electron oxygen reduction reaction (ORR) with the assistance of the catalytic activity of Au NP [3]. Thirdly, the HEPI junction-induced shape change of Au NPs is shown to occur on the surface of ZnO in the Au#ZnO system. The extremely high UV-light activity of Au#ZnO for two-electron ORR far exceeding that of Au/TiO2 is discussed in terms of the catalytic activity of Au NPs and the adsorption ability of ZnO for H2O2 [4].

[1] H. Tada, Dalton Trans.
2019, 48, 6308, and the references therein.

[2] K. Awa, R. Akashi, A. Akita, S. Naya, H. Kobayashi, H. Tada, ChemPhysChem
2019, 20, 2155.

[3] K. Awa, S. Naya, M. Fujishima, H. Tada, J. Phys. Chem. C
2020, 124, 7599.

[4] M. Teranishi, S. Naya, H. Tada, J. Am. Chem. Soc.
2010, 132, 7850.
(Invited) Fermi Level Pinning Controls Photochemical Charge Separation in Particles of n-SrTiO3, n-SrTiO3:Al, and p-GaAs:Te
Authors: Osterloh Frank , Doughty Rachel M , Hodges Brett , Dominguez Julius Esmerald , Han Ruirui , Zhao Zeqiong , Assavachin Samutr
Published: December 2020

The space charge region (SCR) is a majority-carrier-depleted region near the interface of a semiconductor. The width of this region and the size of the potential drop across it influence the charge separation at the semiconductor interface. The space charge region is difficult to evaluate for photocatalyst particles due to their small dimensions and poor electric contact. Surface Photovoltage Spectroscopy (SPS) is a highly sensitive spectroscopic technique which can be used to monitor charge transfer and polarization in particle films. Here we apply SPS to observe the SCR in thin films of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) particles on gold substrates. Negative photovoltages correspond to light-induced majority carrier injection into the gold substrate while positive photovoltages correspond to majority carrier repulsion away from the particle-gold interface. From the inverted photovoltage size, potential barriers are estimated between 0.005 V and 0.189 V and SCR widths between 2.4 μm and 2.8 μm. The small size of the observed barriers and the large SCR widths are attributed to charge trapping in particle surface states (Fermi Level pinning), likely from chemisorbed oxygen. This suggests that depletion layers in particles are mainly controlled by surface states.

Figure 1
(Invited) Synthesis and Integration of Wide Bandgap Chalcopyrite Candidates for Photoelectrochemical Water Splitting Applications
Authors: Gaillard Nicolas
Published: December 2020

Photoelectrochemical (PEC) water splitting has the potential to become an efficient method for renewable hydrogen production. However, the efficiency, projected cost, and durability of lab-scale systems are not yet at the level required to make this technology economically feasible. Amongst all materials studied to date, the chalcopyrite class is arguably one of the most promising classes for PEC water splitting, as it has already demonstrated low-cost and high photoconversion capabilities as a photovoltaic material. In the context of PEC, our group has shown that co-evaporated 1.65 eV bandgap (EG) CuGaSe2 is capable of evolving hydrogen with Faradaic efficiency greater than 85% and generate photocurrent densities over 15 mA.cm-2, as measured in a 3-electrode configuration in 0.5M H2SO4 under simulated AM1.5G illumination. Unfortunately, CuGaSe2’s narrow EG limits its integration as top absorber into a dual junction stacked PEC device (also known as hybrid photoelectrode, HPE). In the present communication, we report on our latest efforts to synthesize wide-EG (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating saturated photocurrent densities greater than 10 mA/cm2. We present specifically results on EG tunable Cu(In,Ga)(S,Se)2, Cu(In,Ga)S2, CuGa(S,Se)2 and CuGa3Se5. We also discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of bandgap tunable MgZnO n-type buffer layers. We also include recent durability testing of chalcopyrite photocathodes coated with metal oxide protection layers. Finally, we present solid-state techniques and theoretical modeling used by our team to identify possible pitfalls in these material systems and discuss potential paths for improvement.
(Invited) SnS2 Thin Film and Powder for Artificial Photosynthesis
Authors: Chen Kuei-Hsien , Shown Indrajit , Chen Li-Chyong , Reta Tadesse Billo , Wu Yu-Kai
Published: December 2020

Solar photoreduction of CO2 to produce value added hydrocarbons is highly desirable to tackle environmental and energy issues. [1-2] Despite the great improvement in the efficiency and cost of solar cells, the efficiency in artificial photosynthesis is much lower than the >15% of solar cells. Recent progress in 2D chalcogenides with tunable bandgap and layer numbers offers great opportunity for the investigation in this field. In this work, thin film and powder SnS2 have been synthesized for the study. Hydrothermal synthesis of carbon-containing SnS2 exhibits a highly active photocatalytic conversion of CO2 to selective hydrocarbons under visible-light irradiation. Overall, the carbon doping in the SnS2 nanostructure plays a key role and significantly enhance the visible light photocatalytic activity with a photochemical quantum efficiency above 0.7%. [3-4] Meanwhile, Thin film SnS2 has been grown for the mechanism study. By carbon-ion implantation of the thin film, we observed enhanced CO2 reduction under solar illumination. Other FTIR, Raman, NMR, BET, SECM, and synchrotron based facilities including APXPS and XAS have been used to investigate this issue.

1. Shown, H.C. Hsu, Y.C. Chang, C.H. Lin, P.K. Roy, A. Ganguly, C.H. Wang, J.K. Chang, C.I. Wu, L.C. Chen, K.H. Chen, Nano Letters 14, 6097-6103 (2014).

2. Billo, F.Y. Fu, P. Raghunath, I. Shown, W.F. Chen, H.T. Lien, T.H. Shen, J.F. Lee, M. C. Lin, J.S. Hwang, C.H. Lee, L.C. Chen, and K.H. Chen, Small 14, 1702928 (2018).

3. Shown, S. Samireddi, Y.C. Chang, R. Putikam, P.H. Chang, A. Sabbah, F.Y. Fu, W.F. Chen, C.I. Wu, T.Y. Yu, P.W. Chung, M.C. Lin, L.C. Chen, and K.H. Chen, Nature Comm. 9, 169 (2018).

4. T. Billo, I. Shown, A.K. Anbalagan, T.A. Effendi, A. Sabbah, F.Y. Fu, C.M. Chu, W.Y. Woon, R.S. Chen, C.H. Lee, K.H. Chen, L.C. Chen, Nano Energy 72, 104717 (2020).
Plasmon-Mediated Photochemical Reactions
Authors: Lin Yan-Gu
Published: December 2020

The Cu2O-based photocathode is considered as one of the best performing photocathode materials for solar water reduction. However, the relatively negative onset potential for H2 production of these photocathodes impedes further optimization of the solar-to-fuel conversion efficiency. An important reason is the inability to achieve meaningful photovoltages with Cu2O. Here, the photovoltage barrier can be readily broken by replacing the semiconductor/water interface with a semiconductor/semiconductor one. A n-Cu2O layer was found to form high-quality buried junction with p-Cu2O to increase the photovoltage and thus shift the turn-on voltage positively. Mott−Schottky Measurements confirmed that the improvement was benefited from a thermodynamic shift of the flatband potentials. An extremely positive onset potential in n-Cu2O/AuAg/p-Cu2O was further obtained, which is comparable to what has been measured using water reduction catalysts. The alloy nanoparticles sandwiched between the homojunction of n-Cu2O and p-Cu2O play an important role in enhancing the photocatalytic performance. First, the alloy nanoparticles not only serve as an electron relay, but also promote electron-hole pair generation in nearby semiconductors, which facilitates the charge transfer between n-Cu2O and p-Cu2O since the sandwich structure is measured by X-ray absorption spectroscopy. Second, the alloy nanoparticles act as a plasmonic photosensitizer, which enables the solar-to-hydrogen conversion at wavelengths longer than the band edge of homojunction structure, extending the incident photo-to-current conversion efficiency wavelength. Finally, the plasmonic energy-transfer mechanism is identified as direct transfer of the plasmonic hot carriers, and the interfacial Schottky barrier height is shown to modulate the plasmonic hot electron transfer. This facile sandwich structure combines both the electrical and the optical functions of alloy nanoparticles into a single structure, which has implications for the design of efficient solar-energy-harvesting devices.
Photoelectrochemical Nitrogen Fixation for Ammonia Synthesis Using Hybrid Plasmonic Nanostructures
Authors: Nazemi Mohammadreza , El-Sayed Mostafa
Published: December 2020

Cost-effective production of ammonia via (photo)electrochemical nitrogen reduction reaction (NRR) hinges on N2 electrolysis at high current densities with suitable selectivity and activity [1, 2]. In this talk, we report our findings in electrochemical NRR for ammonia synthesis using porous bimetallic Pd-Ag nanocatalysts in both gas-phase and liquid-phase electrochemical cells at current densities above 1 mA cm-2 under ambient conditions. While the gas-phase cell has lower Ohmic losses and higher energy efficiency, the liquid-phase cell achieved higher selectivity and Faradaic efficiency, attributed to the presence of concentrated N2 molecules dissolved in an aqueous electrolyte and the hydration effects. The liquid cell demonstrated notable performance for electrocatalytic NRR, achieving an NH3 production rate of 45.6 µg cm-2 h-1 at a cathodic potential of -0.6 V (vs. RHE) and current density of 1.1 mA cm-2, corresponding to a Faradaic efficiency of ~19.6% and an energy efficiency of ~9.9%. Similarly, the gas-phase cell achieved an NH3 yield rate of 19.4 µg cm-2 h-1 at -0.07 V (vs. RHE) and 1.15 mA cm-2 with a Faradaic efficiency of 7.9% and an energy efficiency of 27.1%. In addition, the photoelectrocatalytic activities of these hybrid plasmonic nanostructures under illumination and dark conditions will be explored and photocurrent and photovoltage responses will be reported. Operando surface-enhanced Raman spectroscopy (SERS) is used to identify the intermediate species relevant to NRR at the solid-liquid (electrode-electrolyte) interface. This work highlights the importance of design and optimization of cell configuration in addition to the modification of the catalyst to achieve high-performance N2 electrolysis for ammonia synthesis. It also demonstrates the use of operando SERS as a powerful technique for unraveling reaction mechanisms for (photo)electrocatalytic phenomenon.

[1] M. Nazemi, L. Soule, M. Liu, M. A. El-Sayed, “Ambient Ammonia Electrosynthesis from Nitrogen and Water by Incorporating Palladium in Bimetallic Gold-Silver Nanocages” Journal of The Electrochemical Society, 167(5), p. 054511, 2020.

[2] M. Nazemi, M. A. El-Sayed, “Plasmon-Enhanced Photo(electro)chemical Nitrogen Fixation under Ambient Conditions Using Visible Light Responsive Hybrid Hollow Au−Ag2O Nanocages” Nano Energy, vol. 63, 103886, 2019.
(Invited) Thermoplasmonic Generation of Carbon Nanohydrids for Enhanced Electrocatalysis
Authors: Naldoni Alberto
Published: December 2020

Local heating from plasmonic nanostructures has shown great potential in photothermal therapy, optical trapping, heterogeneous catalysis, and synthesis of nanostructures. Titanium nitride (TiN) is one the most promising plasmonic materials for thermal applications (thermoplasmonics) due to its high thermostability (i.e.

refractory), CMOS compatibility, and resistance to chemically harsh environments. On the other hand, hydrogen peroxide (H2O2) has a large annual production approaching 4 ×106 tons worldwide and the selective reduction of O2 via electrochemical oxygen reduction reaction (ORR) using renewable electricity is a promising way for H2O2 production. Carbon-based materials have shown great potential for the two-electron ORR to hydrogen peroxide. In this contribution, I will show a general approach to use the highly intense local temperature generated from plasmonic TiN nanocrystals for generating TiN/carbon nanohybrids that show enhanced efficiency and selectivity toward the ORR. The general applicability of plasmon-driven synthesis of nanocarbons to electrocatalysis will be discussed.
(Invited) Ultrafast X-Ray Studies of Electron Dynamics Underlying Molecular and Nanoplasmonic Light Harvesting Architectures
Authors: Gessner Oliver
Published: December 2020

Chemically engineered light harvesting architectures based on molecular and nanoplasmonic chromophores in contact with semiconducting electron acceptors provide exciting opportunities for novel photocatalytic and photovoltaic applications. Optimizing their performance, however, often hinges on a deeper understanding of the underlying fundamental electronic and chemical dynamics. Processes evolving on spatial and temporal scales spanning many orders of magnitude have to be connected in order to enable molecular, interfacial, and macroscopic charge and energy transport. We will discuss the application of ultrafast X-ray spectroscopy techniques to gain deep insight into photoinduced electronic dynamics in complex, heterogeneous light-harvesting systems based on organic heterojunctions as well as molecule-semiconductor and metal nanoparticle-semiconductor interfaces.

Femtosecond and picosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) is used to study photoinduced charge injection in N3 dye-sensitized films of ZnO nanoparticles. The elemental site-specificity and surface sensitivity of TRXPS provides a comprehensive real-time picture of the interfacial charge dynamics including the locations of intermittently trapped electrons, the emergence and decay of interfacial dipoles, as well as charge-injection induced band-bending dynamics in the semiconductor substrate. Potential consequences of the findings for the design of dye-sensitized nanoporous electrodes are discussed.

TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of the predominant energy transport and charge generation mechanisms. Ultrafast X-ray probing captures the actual moment of charge generation from interfacial charge-transfer (ICT) states and provides a quantitative measure of the exciton dissociation efficiency compared to photon energy loss by exciton recombination. The findings complement observations with ultrafast optical techniques, adding important aspects regarding the dynamic pathways and branching ratios that govern charge generation efficiencies in this canonical model system for organic light harvesting concepts.

In a third showcase example, photoinduced charge transfer dynamics in a nanoporous film of TiO2 sensitized with gold nanoparticles (AuNPs) is monitored by TRXPS. Direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics is demonstrated with single-electron sensitivity. The results provide important boundary conditions for the design of AuNP-TiO2 based light harvesting systems. Opportunities for extending the measurements toward studies of photoinduced interfacial chemical transformations will be discussed.
Photoelectrochemical CO2 Reduction Using CuInS2 Photocathode Modified with CdS and Noble Metal Catalysts
Authors: Fujishiro Yukitaka , Takashima Toshihiro , Irie Hiroshi
Published: December 2020

Photoelectrochemical CO2 reduction using sunlight is a promising approach to producing a clean energy carrier that could replace fossil fuels, providing a sustainable future energy supply. Copper chalcopyrite compounds, Cu(In,Ga)(S,Se)2, have recently received considerable attention as promising candidates for photocathodes owing to their high absorption coefficients and high carrier mobilities, and efficient photoelectrochemical H2 production using a series of Cu(In,Ga)(S,Se)2-based photocathodes has been demonstrated. Moreover, the photoelectrochemical performance of these photocathodes has been reported to improve when combined with n-type semiconductor buffer layers due to the formation of a p–n junction, which facilitates charge separation at the interface. Meanwhile, noble metal catalysts have been widely used to catalyze redox reactions efficiently because they can lower the overpotential for desirable reactions. Especially, some Au and Ag catalysts have been reported to proceed highly efficient and selective electrochemical CO2 reduction. Based on these backgrounds, in this study, we investigated photoelectrochemical CO2 reduction using CuInS2 electrode modified with CdS buffer layer and noble metal catalysts.

CuInS2 thin film electrodes were synthesized on Mo foil using electrodeposition and subsequent sulfurization. The electrode surface was modified with CdS by chemical bath deposition method, followed by photoelectrochemical deposition of Pt, Au, and Ag. As shown in Figure, the prepared photoelectrodes showed distinct photocurrent generation under visible light irradiation. Modification with noble metal catalysts was effective to enhance the photoresponse and the photocurrent density increased in the order of Pt > Au > Ag. As a result of photoelectrolysis at -0.1 V vs RHE in CO2-saturated NaHCO3 solution, generation of CO was confirmed from all the electrodes although the Faradaic efficiency was different depending on the noble metal catalysts. Details will be discussed at the conference.

References

[1] W. Septina, Gunawan, S. Ikeda, T. Harada, M. Higashi, R. Abe, M. Matsumura, J. Phys. Chem. C 119 (2015) 8576–8583.

[2] Gunawan, Wilman Septina, Shigeru Ikeda, Takashi Harada, Tsutomu Minegishi, Kazunari Domen, Michio Matsumura, Chem. Commun., 50 (2014), 8941

[3]Weichum Ye, Junfeng Yan, Qian Ye, Feng Zhou J. Phys. Chem. C., 114 (2010), 15617

Figure 1
Tandem Solar Device Integration Using Transversely Conductive Polymer Composites
Authors: Crunk Joshua , DeAngelis Alexander D , Gaillard Nicolas
Published: December 2020

In the presented communication, a transversely conductive polymer composite (CC) was designed for electrically coupling multiple solar cells into a tandem device. The CC is composed of epoxy resin and silver coated poly(methyl methacrylate) (Ag-PMMA) microspheres. Three variations of using this CC were investigated: (i) fully-bonded, (ii) semi-bonded, and (iii) freestanding. A fully-bonded CC permanently bonds two solar devices in series. These tandem devices have thus far shown great promise in our lab exhibiting VOC > 1 V for silicon/chalcopyrite tandem devices. The semi-bonded CC encapsulates and protects a solar device (e.g. photoelectrochemical electrode) while allowing electrical conduction through the encapsulation. On-going research on this front has led us to attempt the fabrication of fully immersed water-resistant perovskite PEC electrodes. Lastly, freestanding CCs can be employed to couple solar devices that have been independently fabricated without permanently bonding the cells. The series resistance ranged from 10-1 – 100 Ω∙cm-2. Transmissivity of the CC resulted in measured transmittance over 90% across the visible spectrum.