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Sn-Cu Codeposition from Non-Aqueous Solvents for Wafer Bonding Applications: A Pulse Plating Study
Authors: Pallaro Mattia , Panzeri Gabriele , Magagnin Luca
Published: February 2020

Eutectic wafer bonding is being investigated in industry and research due to the possibility to work at relatively low temperature and, at the same time, to obtain a metallic junction that offers greater reliability compared with other materials, such as polymers or glass[1]. In this work, electrodeposition of Sn-Cu alloy suitable for wafer bonding is studied using ethylene glycol as a solvent: this non-aqueous solvent is used for its promising properties, such as a wider electrochemical window in comparison with commercial solutions[2,3]. Several Sn-Cu solutions are tested with different concentrations of the two metal precursors and are characterized electrochemically, by cyclic voltammetry and linear sweep voltammetry. Deposition is performed both in direct and pulsed current mode, and the advantages of the latter are observed. The effects of pulse plating on the microstructure of the deposits are then investigated, by changing parameters as duty cycle, frequency and the shape of the pulses.

Figure 1 (see attachment): SEM morphologies of two samples deposited in direct current (a) and in pulsed current mode (b).

[1] C. C. Lee, C. Y. Wang, and G. S. Matijasevic, “A new bonding technology using gold and tin multilayer composite structures,” IEEE Trans. Compon. Hybrids Manuf. Technol., vol. 14, no. 2, pp. 407–412, Jun. 1991.

[2] H. M. Maltanava, T. N. Vorobyova, and O. N. Vrublevskaya, “Electrodeposition of tin coatings from ethylene glycol and propylene glycol electrolytes,” Surf. Coat. Technol., vol. 254, pp. 388–397, Sep. 2014.

[3] A. P. Abbott, A. I. Alhaji, K. S. Ryder, M. Horne, and T. Rodopoulos, “Electrodeposition of copper–tin alloys using deep eutectic solvents,” Trans. IMF, vol. 94, no. 2, pp. 104–113, Mar. 2016.

Figure 1
Influence of Molar Ratio of CaO to TiO2 on Ti Electrodeposition in Molten CaCl2
Authors: Okada Haruka , Murata Maito , Morishige Taiki , Takenaka Toshihide
Published: February 2020

A new efficiency process of Ti production replacing Kroll process is desired. The authors proposed the direct electrowinning of liquid Ti considering its advantage in term of productive efficiency. Our previous study showed that calcium titanate was dissolved in molten CaCl2, and that the type of calcium titanate affected the reduction behavior of Ti. These results indicated that Ti reduction in molten CaCl2 depends on the molar ratio of CaO to TiO2 (RCaO/TiO2, hereinafter).

In this study, the influence of RCaO/TiO2 on Ti reduction reaction was investigated by cyclic voltammetry at 1373K ~ 1573K using the bath of RCaO/TiO2 = 1.0 ~ 2.0, and electrodeposition by potentio-static electrolysis was attempted. Typical cyclic voltammograms in the bath of various RCaO/TiO2 were shown in Fig.1. Although no sharp current peak was seen, some current peak/humps appeared; three current peak/humps were observed in the bath of RCaO/TiO2 = 1.0 and 1.325, while two current peak/humps were seen in the bath of RCaO/TiO2 = 1.5, 1.8 and 2.0. These results suggest that stable Ti-O complex ions change with RCaO/TiO2, and that they affect the Ti reduction.

Titanium metal was obtained by potentio-static electrolysis regardless of RCaO/TiO2, but the situation of Ti metal deposition depended on RCaO/TiO2 and the electrolysis potential; Ca metal was apt to co-deposited with Ti metal deposition in the bath of RCaO/TiO2 = 1.0 and 1.325, while Ti metal was obtained without Ca co-deposition in the bath of RCaO/TiO2 = 1.5, 1.8 and 2.0. The suitable electrolytic potential for Ti metal deposition without Ca co-deposition seemed around -1.8V (vs. Mo dissolution potential).

From the reseachers by cyclic voltammetry and potentio-static electrolysis, it was shown that the electrodeposition of Ti metal in molten CaCl2 containing TiO2 strongly depended on the bath composition and the electrolytic potential deposition. The suitable molar ratio of CaO to TiO2, RCaO/TiO2, in the bath is thought to be around, and the suitable potential was -1.8V.

Figure 1
Tunable Reversible Ionic Liquids: Structurally Determined Property Manipulation
Authors: Biddinger Elizabeth J , Jung Sungyup , Podder Showmik , Chen Josephine
Published: February 2020

Reversible ionic liquids (RevILs) are a class of solvents which are promising for a range of applications. RevILs have shown structurally dependent properties by changing their functional group and exhibited dramatic property changes between their molecular liquid (ML) and ionic liquid (IL) states. These unique characteristics of RevILs suggest a method of creating a suite of designer solvents that meet various chemical applications[1, 2]. The concept of “reversibility” lies in the ability of the RevIL to switch from ML to IL with CO2 addition and to switch back from IL to ML with a mild heating, showing dramatic difference of properties [1]. Non-electrochemical applications using RevIL systems include CO2 capture and separation [1].

The RevIL will exist in the ionically-insulating and relatively non-polar ML state, as synthesized, while IL state can be ionically conducting and relatively polar compared to the ML. Additionally, the RevIL properties will change as different functional groups are introduced. The tunability and switchability of the RevILs also make them promising for alternative uses in electrochemical systems[3].

This work investigates the feasibility of RevIL switchable solvents for use as switchable electrolytes in electrochemical applications. Silylamine RevILs are used in this study, and their properties as switchable electrolytes including conductivity, viscosity, and reversibility are primarily investigated. We will also show the change of properties as functional group changes. The change in conductivity and viscosity have signaled the possibility of implementing silylamines as a co-solvent in battery electrolytes to serve as a thermally-activated safety switch. Because these electrolytes have phase change from IL to ML with polarity and conductivity change at reversal temperature, the reversal temperature has also been studied to provide information about the temperature dynamics of the IL.

[1] J.R. Switzer, A.L. Ethier, E.C. Hart, K.M. Flack, A.C. Rumple, J.C. Donaldson, A.T. Bembry, O.M. Scott, E.J. Biddinger, M. Talreja, M.G. Song, P. Pollet, C.A. Eckert, C.L. Liotta, Design, synthesis, and evaluation of nonaqueous silylamines for efficient CO2 capture, ChemSusChem, 7 (2014) 299-307.

[2] P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L. Liotta, Reversible nonpolar-to-polar solvent, Nature, 436 (2005) 1102.

[3] J.D. Jimenez, S. Jung, E.J. Biddinger, Ionicity analysis of silylamine-type reversible ionic liquids as a model switchable electrolyte, Journal of the Electrochemical Society, 162 (2015) H460-H465.
Negative Pressure Regimes in Ionic Liquids: Structure and Interactions in Stretched Liquids as Probed by NMR
Authors: Silva Wagner , Veiga Helena Isabel M. , Tariq Mohammad , Cabrita Eurico J. , Esperança José M.S.S. , Canongia Lopes José N. , Rebelo Luis Paulo N.
Published: February 2020

Condensed matter - solids or liquids, but not gases - might be deeply, homogeneously stretched to absolute negative pressures (metastable regimes of isotropic tension). For instance, we were able to stretch macroscopically-sized liquid solutions down to p ~ - 35 MPa,[1] a value which still constitutes a world record for large volume liquid samples. Up to now, the absolute record for neat liquids is hold for liquid water in microscopic porous crevices of natural rocks, p ~ -140 MPa.

There are several strategies to force a liquid entering into negative pressure regimes. The most common one is the Berthelot method.[2] Negative pressures can be either directly measured - via a highly elaborated Bourdon-Berthelot combination methodology[3,4] or safely, though approximately, estimated through the liquid’s isochoric thermal-pressure coefficient, g
v = a
p/k
T.

In this work, a broad pallete of macroscopically sized samples of ionic liquids was used, in which tensions of about -100 MPa were obtained before cavitation would occur. This value constitutes a new record for large samples. The underlying success for achieving such great absolute negative pressures in ionic liquids as compared to other liquids seems to be the best combination of (i) their negligible vapour pressure and (ii) their generally high viscosity. Good adhesion (wettability) to the container’s walls (in this case Pyrex glass) is a paramount, minimum requirement.

For the first time, NMR studies have been performed on ionic liquids under homogeneous tension. Due to potential hazard issues related to the protection/safety of the NMR instrumentation, negative pressures beyond the value of – 20 MPa were avoided. Upon entering into these metastable regimes of negative pressure, it was possible to observe a sharp increase in the ions self-diffusion coefficients as compared to the equilibrium saturated liquid conditions at the same temperatures.

MD simulations have been performed to gain insights into the ILs structure and interactions under these deep metastable conditions.

References:

P. Visak, L.P.N. Rebelo, J. Szydlowski, J. Phys. Chem. B 107 (2003) 9837-9846.

Imre, K. Martinás, L.P.N. Rebelo, J. Non-Equil.Thermodyn. 23 (1998) 351-375.

I.M. Veiga et al., Int. J. Thermophys. 22 (2001) 1159-1174.

P.N. Rebelo et al., in “Liquids Under Negative Pressure”, Eds.: A.R. Imre, H.J. Maris, and P.R. Williams, NATO Science Series, Vol. 84, Kluwer Academic Publishers, Dordrecht, pp. 95 – 108 (2002).
Photo-polymerized organic host network of ionogels for lithium batteries: effects of mesh size and of ethylene oxide content
Authors: Aidoud Djamel , Guy-Bouyssou Delphine , Guyomard Dominique , Le Bideau Jean , Lestriez Bernard
Published: February 2020

Ionogels represent a route to biphasic materials, for the use of ionic liquids (ILs) for all-solid devices. Confining ILs within host networks enhances their averaged dynamics, resulting in improved charge transport. Fragility, short relaxation times, low viscosity, and good ionic conductivity, all them appear to be related to the IL / host network interface. The presence of ILs at interface neighborhood leads to the breakdown of aggregated, structured regions that are systematically found in bulk ILs. This “destructuration”,1 as well as segregative interactions at interface,2 coupled with percolation of the bicontinuous solid/liquid interface,3 make these materials very competitive among the existing solid electrolytes. Such approach could provide (i) a route to lower locally the viscosity of ILs, and (ii) an easier pathway for diffusion of charged species. Several types of ionogels demonstrate this effect, taking into account of fully inorganic, hybrid, polymeric or organic-inorganic host networks . This “all solid” approach can be applied to several electrochemical energy storage sources, including lithium batteries 2,4 and supercapacitors.5

Herein we will emphasize the results of systematic studies of the effect of size of confinement, either within pores of solids or within meshes of polymers.2,6 Besides the effect of the size of the confinement, the chemical nature of the host network is investigated. Several examples showed that the best properties were obtained on the basis of a good balance between features of host network and features of confined ionic liquid.

References:

[1] A. Guyomard-Lack et al., Phys. Chem. Chem. Phys., 16 (2014) 23639-23645.

[2] A. Guyomard-Lack et al., New J. Chem., 40 (2016) 4269-4276.

[3] C. V. Cerclier et al., Phys. Chem. Chem. Phys., 17 (2015) 29707—29713

[4] D. Aidoud et al., J. Power Sources, 330 (2016) 92-103

[5] M. Brachet et al., J. Mater. Chem. A, 4 (2016) 11835-11843.

[6] D. Aidoud et al., submitted.
(Invited) Electrochemical Synthesis of Metal and Semiconductor Nanostructures from Ionic Liquids
Authors: Borisenko Natalia
Published: February 2020

Metal and semiconductor nanostructures are usually synthesized using vacuum techniques. Such processes are cost-intensive and need vacuum conditions for preparation thus making the semiconductors quite expensive. Electrodeposition is an alternative and relatively simple way to produce these materials. However for electrochemical synthesis, applying of a proper solvent is essential to produce metals and semiconductors. The electrodeposition from aqueous solutions is limited by a relatively low electrochemical stability of water. In the case of molten salts, high temperature is usually required for electrodeposition. Organic solvents are very volatile and many of them are quite toxic. The alternative to common electrolytes is to use ionic liquids (ILs), which exhibit wide electrochemical windows, good thermal stability, non-volatility, low toxicity and are liquid below 100 °C [1].

In this lecture we will show that ILs are well suited for the electrodeposition of metal and semiconductor nanostructures with various architectures. Consequently, metal (e.g. Ga, In, Zn, Al, Cu, etc.) and semiconductor (Si, Ge, SixGe1-x, etc.) nanowires, nanotubes and macroporous structures can be synthesized in ILs at room temperature by using template-assisted electrodeposition. Recently, these materials have received some attention as promising electrode materials in batteries [2]. Furthermore, III-V semiconductor (e.g. GaSb and InSb) nanowires can be directly synthesized at room temperature by electroless deposition [3]. Some nanowires (e.g. ZnS) can be obtained in ILs without any external template, which opens a new perspective for the synthesis of semiconductor nanostructures. The structure of the IL/electrode interface can significantly alter the deposition process that, in turn, may result in the deposits with various morphologies and properties. This offers the opportunity to influence the optical properties of obtained materials by changing the composition of the solvent.

[1] F. Endres, A. Abbott, D. R. MacFarlane, Electrodeposition from Ionic Liquids. 2nd Edition. 2017, Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA.

[2] M. Shapouri Ghazvini, G. Pulletikhurthi, Z. Liu, A. Prowald, S. Zein El Abedin, F. Endres, J. Solid State Electrochem., 19 (2015) 1453-1461.

[3] A. Lahiri, N. Boriseko, M. Olschewski, R. Gustus, J. Zahlbach, F. Endres, Angew. Chem. Int. Ed., 54(40) (2015) 11870-11874.
Influence of Additives on Aluminum (Al) Plating from Ionic Liquids
Authors: Li WeiNa , Zhang Weilong , Abbott Joshua G.
Published: February 2020

Aluminum (Al) plating from ionic liquids has attracted both scientific and technological interest because it provides an eco-friendly, and drop-in replacement of cadmium as a corrosion protection coating. Considerable effort has been made to advance this ionic liquid-based Al plating process as a competitive commercial aluminum deposition technology. With the aim of the industrialization of the Al plating process, the use of additive-free electrolyte is preferred from ionic liquid plating bath durability. However, like conventional plating from aqueous solution, certain additives are required for smooth and high quality aluminum coatings from ionic liquids. Theoretically, these additives act as leveling or brightening agents to improve the quality of deposits but often at the cost of coating purity, and coating properties such as corrosion resistance. Therefore, a better scientific understanding of different additives on Al plating from ionic liquids from both Al plating process and Al coating performance would be beneficial to advance this commercially viable Al plating process from ionic liquids. In this work, commercially available ionic liquids, composed of both [Emim]Cl/AlCl3 and [Bmim]Cl/AlCl3 systems with different proprietary additives from suppliers were employed for the influence of additives on Al plating on both Cu and steel substrates. The resultant surface morphologies and adhesion and corrosion resistance of the Al coatings were evaluated, and further characterized by FIB/SEM/EDXS, demonstrating that all proprietary additives serve as a highly effective brightener, producing highly uniform and smooth Al deposits, but could lead to different Al coating performance.
Hot Corrosion Study of TiO2-ZrO2 Coating on Inconel 601 Alloy in NaVO3 Molten Salt
Authors: Espinoza-Villa Sergio , Carbajal-De la Torre Georgina , Cadenas Erasmo , Espinosa-Medina Marco Antonio
Published: February 2020

The corrosion resistance effect of a ceramic coating applied on –Inconel-601 alloy immerse in molten salts was studied. The development of ceramic coating was obtained by sol-gel synthesis of TiO2-ZrO2, FTIR analysis was used to characterize the resultant sol. The dip-coating technique was used to coating application using the sol solution and Inconel-601 substrate set previously in an alumina tube with the cylindrical side exposed. The molten salt of metavanadate of sodium (NaVO3) at 700 °C was used as corrosive electrolyte, the electrochemical cell consisted uncoated and coated samples as working electrode and two platinum wire as references and auxiliary electrodes. Those last were placed inside to alumina tubes in order to isolate the non-immersed part from air. Potentiodynamic long polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of samples. Potentiodinamic test were applied at -500 to 1500 mV vs open corrosion potential (OCP) with a 1 mV/s scan rate. LPR was realized within a ± 15 mV vs OCP at 1 mV/s scan rate each 15 minutes during 24 h. The EIS was realized in the frequency of from 25 kHz to 0.01 Hz with a potential signal amplitude of 30 mV vs OCP. Potentiodynamic results did not showed a substantial difference on the current densities, but some passive steps were presented by the coated sample, however the coated sample showed a higher corrosion resistance kinetic than uncoated sample with average values 160 ohm cm2 and 40 ohm cm2 respectively. EIS results present the active mechanism of the uncovered Inconel at the first time of immersion followed by diffusion increasing with the corrosion products, however coated sample showed a mixture of capacitive and diffusion mechanism since the first time of immersion with higher resistance values as RPL results. Those results were complemented by the cross section characterization by SEM and EDS, showing a reduction of salt species penetration from 48.5 to 21.3 µm in deep. Chemical mapping was obtained also. Scanning electron microscopy observation and chemical analysis by energy dispersive spectroscopy were used to characterize the as received and after corroded materials in order to complement the corrosion behavior study.

Keywords: TiO2-ZrO2 coating; Inconel 601 alloy; electrochemical techniques; NaVO3 Molten Salt

Acknowledgements

The present research was supported by CONACYT for the project financing number: CB-2014-01-243236, I0017 Fund.
Thermal Analysis of the System NaCl–CaCl2–CaO
Authors: Savchuk Ruslan N. , Gritsai Larisa V. , Blyznyuk Antonina V , Skryptun Igor N
Published: February 2020

The property of the systems MCl-CaCl 2 and MCl-CaCl2-CaO (where M is a alkali metal) allow its used in many processes of modern industry, in particular for the production of metals. However, in spite of this the information on the diagrams of the state of ternary systems of MCl-CaCl2-CaO and the behavior of refractory metal oxides in these melt is extremely limited.

This work shows the results of thermal analysis of partial cuts the ternary system NaCl-CaCl2-CaO.

The table 1 shows the compositions and the melting point (crystallization point) characteristic points of the system NaCl-CaCl2-CaO.

Table 1. The composition and melting point (crystallization point) of characteristic points.

System
T °C
Concentrate and I component in , %
The character of the point

Na Cl
CaO
CaCl 2

Na Cl - CaCl 2

500
0.490
-
0.510

Eutectic (
e

1

)

CaO - CaCl 2

750
-
0.065
0.935

Eutectic
(
e

2

)

835
-
0.185
0.715

Peritec
t
ic (
p

2

)

500
0.470
0.05
0.480

Eutectic (
E
)

The results of investigation partial cuts ternary system showing that in the concentration range 0–20 mol.% CaO ternary system NaCl-CaCl2-CaO is eutectic. The composition of eutectic (in mol.%) is NaCl(0.47)-CaCl2(0.48)-CaO(0.05) and its molten point is 480 °C. In the fused samples the phases of the initial components of the system are identified. Solid solutions and phases of the composition Ca(OCl)2 was not detected.
Electrochemical Synthesis of Tantalum Silicides from Chloride-Fluoride Melts
Authors: Devyatkin Sergei Vladimirovich
Published: February 2020

Tantalum silicides: Ta3Si (Tm = 2613 К) , Ta2Si (Tm = 2783 К), Ta5Si3 (Tm = 2833 К) and TaSi2 (Tm = 2313 К) belong to refractory compounds. In the series of refractory compounds, silicides are distinguished by a high chemical stability at high temperatures; therefore, they widely used as protective materials in the chemical industry. Thanks to semiconductor properties and thermal stability, tantalum silicides promise much as a substitute for silicon in microcircuits.

Based on thermodynamic calculation and voltammetric studies, it has been concluded that the synthesis of tantalum silicides is the most electropositive process in the NaCl-KCl-K2SiF6-K2TaF7 melts.

Only the compound Ta2Si as coatings or powders has been obtained by electrosynthesis of the NaCl-KCl-K2SiF6-K2TaF7 melt at 973 K.
Photo-Healable Materials Based on Block Copolymers and Ionic Liquids
Authors: Watanabe Masayoshi
Published: February 2020

Ion gels are composed of ionic liquids (ILs) immobilized within a three-dimensional molecular network. Such gels preserve the attractive physicochemical properties of ILs, such as nonvolatility, thermal stability, electrochemical stability, and high ionic conductivity, while maintaining a soft solid consistency. Ion gels are a novel platform for many applications such as electrolyte membranes, actuators, gas-separation membranes, flexible electrochemiluminescent gels, and organic thin-film transistors.

We present here the reversible micellization and sol-gel transition of block copolymer solutions in an IL triggered by a photostimulus. The ABA triblock copolymer employed consists of a random copolymer A block, including thermosensitive N-isopropylacrylamide (NIPAm) units and a methacrylate incorporating an azobenzene chromophore into the side chain (AzoMA); the B block is composed of an IL-soluble poly(ethylene oxide) (PEO) (P(AzoMA-r-NIPAm)-b-PEO-b-P(AzoMA-r-NIPAm)). We observed a reversible unimer-to-micelle transition of a dilute ABA triblock copolymer (1 wt%) in an IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) induced by photostimuli at an intermediate, “bistable” temperature. We demonstrate a reversible sol-gel transition cycle of the ion gel at the bistable temperature, with reversible association and fragmentation of the polymer network structure consisting of the self-assembled ABA triblock copolymer (20 wt%) in [C4mim]PF6. Such sol-gel transition has been utilized to design photo-healable materials. Damaged part of materials is UV-irradiated to convert gel to sol, followed by irradiated by visible light in order to convert again sol to gel. By this procedure, the damaged part was demonstrated to be healed to the original state (Figure 1).

Furthermore, we propose moving the azobenzene groups from the polymer chains to the IL structure as a molecular-level photoswitchable IL. Photoisomerization of an azobenzene IL at the molecular level can evoke a macroscopic response (light-controlled mechanical sol-gel transitions) for ABA triblock copolymer solutions. Therefore, an azobenzene IL as a molecular switch can tune the self-assembly of a thermoresponsive polymer, leading to macroscopic light-controlled sol-gel transitions. Surprisingly, a sight change in chemical structures of thermosensitive A segments induces opposite photo responsiveness, unimer/micelle transition and sol/gel transition, against UV/Vis-irradiation.

Figure 1. Photographs showing photo-healing of a star shaped ABA ion gel (20 wt%) cut at the center: (Left) damage before applying light stimuli, (Right) completely repaired damaged part after 64 h UV-light exposure, followed by 30 min visible-light exposure. The cracked part is highlighted by white dotted square.

Figure 1
Natural Fiber Welding with Novel Ionic Liquids
Authors: Chung Robert T. , Park Seok , Yates Elizabeth A. , Durkin David P. , Trulove Paul C. , De Long Hugh C.
Published: February 2020

Natural Fiber Welding (NFW) is a versatile biopolymer manipulation process that uses ionic liquids (ILs) to disrupt the intermolecular interactions between polymer strands. NFW enables the reconfiguration of biopolymer material structure as well as the integration of functional materials within the biopolymer matrix. This process is thought to require ILs that have both chaotropic anions and hydrogen bond donating cations, with past work focusing on ILs that have been shown to be solvents capable of the full dissolution of biopolymer materials (e.g. cotton, silk, linen, and keratin). In the present work, we looked to expand the suite of ILs that could be used for NFW by investigating the impact of IL structure and composition on the NFW process with emphasis placed on studying the impact of cation structure. Fluorescently labeled cotton yarns were exposed to selected ILs under controlled time, temperature, and atmospheric conditions. Confocal fluorescence microscopy and scanning electron microscopy were used to evaluate the level of welding in the IL-treated cotton yarns, and atomic force microscopy and tensile strength testing were used to evaluate changes in the physical properties of the welded yarns. Initial results indicate that NFW may not require the use of an IL with a hydrogen bond donating cation as long as a chaotropic anion is present.
(Invited) Ion Clusters and Li Transport in [Pyr12O1][FTFSI] Ionic Liquid-Based Electrolytes
Authors: Giffin Guinevere A , Moretti Arianna , Jeong Sangsik , Pilar Kartik , Brinkkoetter Marc , Greenbaum Steven , Schonhoff Monika , Passerini Stefano
Published: February 2020

The use of highly concentrated, binary ionic liquid/Li salt electrolytes with the novel asymmetric anion FTFSI results in improved rate capability and capacity retention at 20°C, as compared to Li+-diluted systems, in Li-metal and Li-ion cells [1]. This work explores the connection between the bulk electrolyte properties and the molecular organization to give insight into the concentration dependence of the Li+ transport mechanisms. Below 30 mol%, the Li+-containing species are primarily smaller complexes (one Li+ cation) and the Li+ ion transport is mostly derived from the vehicular transport. Above 30%, where the viscosity is substantially higher and the conductivity lower, the Li+-containing species are a mix of small and large complexes (one and more than one Li+ cation, respectively). The overall conduction mechanism likely changes to favor structural diffusion via the exchange of anions in the first Li+ solvation shell [2]. The good rate performance is likely directly influenced by the presence of the larger Li+ complexes, which promote Li+-ion transport (as opposed to Li+-complex transport) and increase the Li+ availability at the electrode. In fact, electrophoretic NMR shows that Li is predominantly migrating in negatively charged Li-anion clusters towards the anode. However, this vehicular transport mechanism has decreasing relevance at elevated Li+ concentrations.

References

G. A. Giffin, A. Moretti, S. Jeong and S. Passerini, J. Power Sources, 2017, 342, 335-341.

G. A. Giffin, A. Moretti, S. Jeong, K. Pilar, M. Brinkkötter, S. G. Greenbaum, M. Schönhoff and S. Passerini, ChemSusChem, 2018, doi:10.1002/cssc.201702288.

M. Brinkkötter, G. A. Giffin, A. Moretti, S. Jeong, S. Passerini and M Schönhoff, Chem Comm, 2018, submitted for publication
(Invited) Solar-Driven Chemical Conversion Using Silicon Photoelectrochemical Cells
Authors: Oh Jihun
Published: February 2020

Solar-driven chemical conversion using photoelectrochemical (PEC) cells has attracted intense interests to produce value-added chemical fuels and feedstock using solar energy. In particular, silicon is attractive for a photoelectrode since it’s earth-abundant and low cost, and has small bandgap to absorb most of solar spectrum. However, silicon suffers from slow kinetics for PEC reactions such as water splitting and CO2 reduction and also from severe corrosion during the operation. In this talk, I’ll present our strategies to develop a high performance and stable silicon-based PEC cells. Firstly, I’ll discuss the light-matter-chemical interaction occurring on a photoelectrode’s surface and demonstrate that PEC reactions are coupled with light abosorption and electrocatalysis bound on the surface of a photoelectrode. Based on these understanding, I’ll present our approaches to maximize light absorption and electrocatalysis in PEC cells (1) by using locally defined 3-dimensional cocatalysts and (2) by using a transparent cocatalysts on Si photoelectrodes. Influences of surface area and coverage of cocatalysts as well as charge transport properties of Si photoelectrodes will be presented using water oxidation reaction as a model chemical reaction. I’ll also show that these approaches can be applicable to CO2 reduction reactions.
(Invited) Sunlight-Assisted Electrocatalytic Productions of Chemicals Coupled with Value-Added Redox Processes
Authors: Park Hyunwoong
Published: February 2020

A novel sunlight-water-energy nexus technology is presented that combines the photoelectrocatalytic (PEC) desalination of saline water and desalination-driven wastewater remediation coupled with the production of molecular hydrogen (H2) from water. To accomplish this, morphologically tailored TiO2 nanorod (TNR) and hydrogen-treated TNR (H-TNR) array photoanodes are placed in an anode cell and Pt foils are located in a cathode cell, while a middle cell containing saline water (0.17 M NaCl) faces these cells through anion and cation exchange membranes, respectively. Upon irradiation by simulated sunlight (AM 1.5G, 100 mW×cm-
2), the photogeneration of charge carriers initiates the transport of chloride and sodium in the middle cell to the anode and cathode cells, respectively, leading to the desalination of saline water. The chloride in the anode cell is converted to reactive chlorine species (RCS), which effectively decompose urea to N2 as a primary product (>80%), while the sodium in the cathode cell accelerates the H2 production from water with a Faradaic efficiency of ~80%. The PEC performance of the H-TNR photoanodes is superior to that of the TNR in the anodic and cathodic processes because of the reduced charge transfer resistance and sub-nanosecond charge transfer kinetics (~0.19 ns), leading to a specific energy consumption of 4.4 kWh×m-
3 for 50% desalination, with an energy recovery of ~0.8 kWh×m-
3. The hybrid system is found to operate for a period of ~60 h with natural seawater, and virtually all the photoanodes are shown to be capable of driving the hybrid process. Although tested as a proof-of-concept, the present technology opens up a novel field involving a sunlight-water-energy nexus, promising high efficiency desalination and the desalination-driven remediation of water with simultaneous H2 production.
Theoretical Investigation of the Catalytic Activity of Goethite (α-FeOOH) for the Electrochemical Water Oxidation
Authors: Lee Tae Hyung , Jang Ho Won
Published: February 2020

Recently, to replace the expansive noble metals in electrocatalyst, transition metals with low cost and comparable performances are chosen as substitutes. Among transition metals, iron, which is one of the most abundant metal in the earth crust, is relatively out of the attention as an electrocatalyst. Due to its earth abundance and well-organized production system, iron has the one-tenth price of nickel, and even one-fiftieth price for cobalt. Considering that the ultimate goal of the water electrocatalysis system is an integrated system with a photovoltaic cell which demands the scale-up processing, the cost competitiveness can be maximized for the commercialization.

To enjoy the strong cost competitiveness of the iron, the poor catalytic property of the iron based electrocatalysts, particularly its low current density should be enhanced. According to several studies, it is known that certain phase, iron oxyhydroxide, can have much higher current density than existing iron-based electrocatalysts. Especially, iron oxihydroxide (FeOOH) belongs to thermodynamically stable form among iron oxides and presented to have enhanced catalytic property by experiments. Though, there are not enough theoretical investigations on its catalytic activity.

Herein, we investigate the water oxidation mechanism on the Iron oxyhydroxide(FeOOH) electrocatalysts using first-principles calculations based on Density Functional Theory. With calculations of relative surface stabilities and adsorbate coverages, the most stable low-index surfaces of FeOOH will be determined. Next, with the determined FeOOH surfaces, we will compare the theoretical overpotentials achieved by calculating each oxygen evolution reaction steps. For each step, the magnetic state will be checked for the explanation of oxygen evolution reaction process. Furthermore, impacts of the various dopants such as Mn, Ni, Co will be also investigated. With the calculation using hybrid functional, the electronic structure of the iron oxyhydroxide, with and without dopants, will be obtained and it will give the explanation of different OER catalytic activity shown by iron oxyhydroxide with different dopants.

For the strict verification of the calculation data, experiment results of the bare and doped FeOOH will be also presented. All the alpha FeOOH samples are synthesized with electrodeposition method and their electrocatalytic properties will be evaluated. With the comparison of calculation and experiment data, it is expected to figure out how the dopants affect bulk properties, reaction at the surface of the FeOOH and eventually influence the electrocatalytic properties.
Fast Flame Annealed Porous CuFe2O4 for Photoelectrochemical Hydrogen Production
Authors: PARK Sangwook , Baek Ji Hyun , Jung Hyun Suk , Zheng Xiaolin
Published: February 2020

Photoelectrochemical (PEC) water splitting is one of the promising renewable energy techniques that can store solar energy into one of the clean and renewable energy resources, hydrogen (H2). However, facilitating the renewable PEC water splitting system on a global scale is still challenging. Especially, finding suitable p-type photocathodes is critical which have a narrow bandgap, a great solar to hydrogen (STH) efficiency and economical fabrication processes with earth abundant elements. Metal oxides have shown remarkable advantages that satisfy the important requirements to be an excellent photocathode among the suggested candidates.

Copper ferrite (CuFe2O4) is a promising but rarely studied p-type photocathode based on the relatively narrow bandgap (~1.7eV) which can utilize a great portion of solar light to realize the maximum STH efficiency of ~30% and photocurrent of ~25mA/cm2 theoretically. In addition, CuFe2O4 has positive flat band and photocurrent onset potentials (~1.0 VRHE) that are critical to achieve high operating photocurrent with n-type photoanode for an ideally unbiased entire solar water splitting system. Moreover, it consists of earth abundant elements. However, there are still limiting features for the use of CuFe2O4 photocathode since it has shown a relatively poor photocurrent and the synthesis process inevitably requires long time annealing at high temperature to produce less defective and better crystalline material for a high charge transport property.

Here, we employed the rapid and high temperature (>980°C) flame as an annealing process for CuFe2O4 photocathode synthesis, demonstrating the significantly enhanced photocurrent (-1.82 mA/cm2 at 0.4VRHE in Ar purged 1M NaOH) as a single material photocathode whose photoactivity is greater than that of most studied photocathodes. There are several unique advantages of the flame annealing compared with other conventional annealing methods for the synthesis. For example, the flame does have negligible ramping time to reach high temperature above 980°C due to its short ignition delay time that finally makes the high crystalline CuFe2O4 in only 16 minutes. Moreover, the flame is operated in the open atmospheric environment and the amount of oxygen can be easily controlled by adjusting the fuel to oxygen equivalence ratio as a combustion parameter. Oxygen rich environment was employed for CuFe2O4 annealing to provide fully oxidized condition for an amorphous Cu and Fe mixture prepared by sol-gel method. Finally, the porosity of CuFe2O4 was highly increased by flame annealing, and the physical structural change positively affects the enhancement of the light absorption property and the specific surface area increase.
Integrated Photoelectrochemical Device for Stable Autonomous Solar Hydrogen Production
Authors: Koonce Jonathan G , Alsaedi Sattar , Cheng Wei , Lee Joun , Mubeen Syed
Published: February 2020

Considerable research has been done by many groups for the creation of standalone, integrated, solar-driven, photoelectrochemical water splitting devices that generate hydrogen using only energy from the sun. However, stability of these devices in the electrolytes needed for the efficient operation of the oxidation catalysts remains a significant challenge for long-term operation. Here, we demonstrate an integrated and truly wireless device for solar water splitting. This device integrates an oxygen evolution reaction (OER) catalyst and a hydrogen evolution reaction (HER) catalyst on a commercially available triple-junction, silicon-based solar cell. Integrated into this system is a transparent polymer coating that allows the solar cell to remain stable under alkaline conditions for over 100 hours of constant operation with one sun (1000 W/m2) illumination. This work also addresses the challenges involved with integrating an earth-abundant metal oxide catalyst on the light absorbing oxidation side of the solar cell, as well as the integration of a low-PGM loaded HER catalyst on the reduction side.
(Invited) Ultrafast Generation and Characterization of Hot Carriers in Plasmonic Nanostructures for Solar Light Harvesting
Authors: Wiederrecht Gary
Published: February 2020

Photoexcitation of plasmonic materials can create energetic electrons and holes that are of strong interest for solar energy conversion, photocatalysis, and light harvesting applications. Since intraband photoexcitation can create energetic (other terminology includes “hot” or “nonthermal”) carriers that are up to the photon excitation energy above the Fermi energy, their potential for increasing the efficiency of optical energy conversion is clear. However, in practice there are several challenges to realizing the potential of hot carriers. First, the efficiency of hot carrier generation is low in many plasmonic materials. Second, the energetic carriers generally thermalize very quickly, on the order of 10-100 femtoseconds, thereby making their extraction before energy loss difficult. Third, the energetic carriers tend to produce small changes in the permittivity of the metal that require advanced spectroscopic experimentation or devices in order to characterize them. In this talk I begin with describing the principles of hot carrier generation and the large advantages of using nanostructured metals to improve their generation efficiency. This is followed by our efforts to spectroscopically characterize hot electrons on an ultrafast timescale, as well as to design new nanostructures for creating hot electrons in greater numbers. Physical insights into how to extract the hot carriers before loss of energy to thermalization is given, and our efforts to understand the anisotropic decay of hot carriers is described. Recent efforts to directly extract the energetic distribution of hot carriers as a function of time are also discussed. I further describe recent experiments on specific photocatalytic processes and new biomimetic nanostructures for light harvesting. These biomimetic structures enable unique opportunities for tuning structure and energy transport properties. This work focuses on the use of chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. A closing description of the research opportunities and capabilities at the Center for Nanoscale Materials user facility is also given. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
(Invited) Surface Plasmon-Driven Water Reduction: Nanoparticle Size Matters
Authors: Wei Wei David
Published: February 2020

Plasmonic-metal/semiconductor heterostructures represent promising photocatalytic constructs for solar-to-fuel energy conversion, but detailed knowledge of the structure-function relationships governing their activity remains elusive. Here, we monitor plasmon-mediated electron transfer (PMET) across Au-TiO2 Schottky junctions at the single-nanoparticle level to clarify the role of metal nanocrystal size in plasmonic photochemistry. Size-dependent photovoltage studies of PMET in Au/TiO2 photoelectrodes reveal that the reduction potential of hot electrons within the TiO2 conduction band is not solely determined by the absolute size of Au nanoparticles, but by the relative size of Au to TiO2. With this new insight, we tailored the physical dimensions of Au/TiO2 photocatalysts to efficiently harvest hot electrons for improved plasmon-driven hydrogen evolution from water. Taken together, these studies elucidate the role of metal nanocrystal size in plasmonic photocatalysis and establish general guidelines for the rational design of plasmonic-metal/semiconductor assemblies that can effectively exploit hot carriers in photochemical reactions.
Visible Light Photocatalytic Production of H2
Authors: Wei Wei
Published: February 2020

It is important to exploit visible light for photocatalysis, but it is still a great challenge. In this talk, our recent efforts for visible light photocatalysis will be highlighted, including novel processes and efficient catalysts. The impressive results for visible light photocatalytic production of H2 will be emphasized.
Novel Photoelectrochemical Fuel Cell System Utilizing Titania Photocatalysts
Authors: De Carvalho Rafaela Cristina , Cassidy John , Betts Anthony
Published: February 2020

In this work, one compartment Photoelectrochemical Cell (PEC) fabricated with inexpensive materials photodegraded selected organic compounds present in model waste waters, with the advantage of producing electrical current. Degradation of pollutants in waste waters by photoeletrocatalysis is well known. The photoelectrocatalytic degradation behaviour of the anti-inflammatory Sodium Diclofenac and others organic contaminants in aqueous solution was studied. TiO2 was utilized as the catalyst under different sources of UV and solar radiance. The catalyst was immobilized in a carbon cloth and in a variety of conductive supports as the photoanode and an air-cathode was utilized to allow an increase in oxygen concentration during the reduction process. By utilizing this photocatalytic process in a Photo Fuel Cell (PFC) device, the rate of organic pollutants destruction may be greatly enhanced, at the same time producing useful electrical energy.

A range of environmental conditions will be explored such as fuel/pollutant concentration level, electrolyte composition and pH variation, in order to determine the best performing catalyst/support/environment combination. A transition metal oxide-based air cathode will also be used (replacing expensive platinum) in conjunction with a modified PFC design, thus characteristics and mechanistic insights will be ascertained as a result.
Solar-Driven Photocatalysis for Wastewater Treatment Promoted by Sulfite Addition
Authors: Deng Wei , Li Ying
Published: February 2020

Solar-driven heterogeneous photocatalysis is considered a promising technique for degradation of organic pollutants in wastewater. Visible light from the solar spectrum is however not efficiently utilized in photocatalysis. In this work, we have developed a sulfite-enhanced visible-light-driven photocatalysis process using BiOBr as the model photocatalyst and methyl orange (MO) as the model organic pollutant. We found that the degradation rate of MO was improved by 29 times in the presence of 20 mM sulfite, and the enhancement increased with the concentration of sulfite. Studies using hole scavengers suggest that sulfite radicals generated by the reactions of sulfite (sulfite anions or bisulfite anions) with holes or hydroxyl radicals are the active species for MO photodegradation using BiOBr under visible light. In addition to the BiOBr/MO system, the sulfite-assisted photocatalysis approach has been successfully demonstrated in a variety of photocatalysis systems including BiOBr/rhodamine B (RhB), BiOBr/phenol, BiOI/MO, and Bi2O3/MO systems under visible light irradiation, as well as in TiO2/MO system under simulated sunlight irradiation. The developed method implies the potential of introducing external active species to improve photocatalytic degradation of organic pollutants.
Renewable Fuels: The European Union Research and Innovation Policies
Authors: Georgiadou Maria
Published: February 2020

Horizon 2020 [1] and its successor Horizon Europe [2] provide the research and innovation framework to support the European Union policies for strengthening its scientific and technological basis in order to deliver on citizen's priorities and shared worldwide values, like fighting global challenges. Actions under the challenge "Secure, clean and efficient energy" of Horizon 2020 answer to the need for cost-effective and secure renewable fuel production to assist the decarbonisation of the energy and transport sectors and contribute to limiting the temperature increase to 1.5°C above pre-industrial levels. Being part of Mission Innovation [3] challenges "Converting sunlight into storable solar fuels" and "Sustainable Biofuels", the European Union leads research in renewable and breakthrough fuel technologies through dedicated actions in the Horizon 2020 Energy Work Programme [4]. Disruptive technologies for renewable fuels that outperform fossil fuels are also included. Dedicated topics on solar fuels and international collaboration opportunities worldwide will be presented. Furthermore, the inducement prize "Fuel from the Sun: Artificial Photosynthesis" which was launched in December 2017 [5] will be covered. Future support under Horizon-Europe will be presented.

References

[1] https://ec.europa.eu/programmes/horizon2020/

[2] http://ec.europa.eu/horizon-europe

[3] http://mission-innovation.net/

[4] http://ec.europa.eu/programmes/horizon2020/en/h2020-section/secure-clean-and-efficient-energy

[5] https://ec.europa.eu/research/eic/index.cfm?pg=prizes#sunfuel