15 results found
Brugge RH, Pesci FM, Cavallaro A, et al., 2020, The origin of chemical inhomogeneity in garnet electrolytes and its impact on the electrochemical performance, Journal of Materials Chemistry A, Vol: 8, Pages: 14265-14276, ISSN: 2050-7488
The interface between solid electrolytes and lithium metal electrodes determines the performance of an all-solid-state battery in terms of the ability to demand high power densities and prevent the formation of lithium dendrites. This interface depends strongly on the nature of the solid electrolyte surface in contact with the metallic anode. In the garnet electrolyte/Li system, most papers have focused on the role of current inhomogeneities induced by void formation in the Li metal electrode and the presence of insulating reaction layers following air exposure. However, extended defects in the solid electrolyte induced by chemical and/or structural inhomogeneities can also lead to uneven current distribution, impacting the performance of these systems. In this work, we use complementary surface analysis techniques with varying analysis depths to probe chemical distribution within grains and grain boundaries at the surface and in the bulk of garnet-type electrolytes to explain their electrochemical performance. We show that morphology, post-treatments and storage conditions can greatly affect the surface chemical distribution of grains and grain boundaries. These properties are important to understand since they will dictate the ionic and electronic transport near the interfacial zone between metal and electrolyte which is key to determining chemo-mechanical stability.
Pesci FM, Bertei A, Brugge RH, et al., 2020, Establishing ultra-low activation energies for lithium transport in garnet electrolytes., ACS Applied Materials and Interfaces, Vol: 12, Pages: 32086-32816, ISSN: 1944-8244
Garnet-type structured lithium ion conducting ceramics represent a promising alternative to liquid-based electrolytes for all-solid-state batteries. However, their performance is limited by their polycrystalline nature and the inherent inhomogeneous current distribution due to the different ion dynamics at grains, grain boundaries and interfaces. In this study we use a combination of electrochemical impedance spectroscopy, distribution of relaxation times analysis and solid state nuclear magnetic resonance (NMR), in order to understand the role that bulk, grain boundary and interfacial processes play in the ionic transport and electrochemical performance of garnet based cells. Variable temperature impedance analysis reveals the lowest activation energy (Ea) for Li transport in the bulk of the garnet electrolyte (0.15 eV), consistent with pulsed field gradient NMR spectroscopy measurements (0.14 eV). We also show a decrease in grain boundary activation energy at temperatures below 0 °C, that is followed by the total conductivity, suggesting that the bottleneck to ionic transport resides in the grain boundaries. We reveal that the grain boundary activation energy is heavily affected by its composition that, in turn, is mainly affected by the segregation of dopants and Li. We suggest that by controlling the grain boundary composition, it would be possible to pave the way towards targeted engineering of garnet-type electrolytes and ameliorate their electrochemical performance in order to enable their use in commercial devices.
Chong JY, Wang B, Sherrell PC, et al., 2019, Fabrication of graphene‐covered micro‐tubes for process intensification, Advanced Engineering Materials, Vol: 21, Pages: 1-6, ISSN: 1438-1656
Graphene is known for its high surface‐area‐to‐mass ratio. However, for graphene to be used in engineering processes such as catalytic reactors or heat exchangers, high surface‐area‐to‐volume ratio is essential. Currently, graphene is only prepared in sheet form, which limits its surface‐area‐to‐volume ratio to around 200 m2 m−3. In this study, we propose and demonstrate a technique based on chemical vapour deposition (CVD) to realise graphene on a copper‐based micro‐tubular substrate to not only substantially increase its surface‐area‐to‐volume ratio to a value over 2000 m2 m−3, but also to eliminate maldistribution of flows commonly unavoidable in flat‐sheet configurations. Our approach uses a dual‐layer micro‐tubular substrate fabricated by a phase‐inversion facilitated co‐extrusion technique. In the substrate, a thin copper outer layer is employed to enable the CVD growth of graphene, and an inner Cu‐Fe layer is adopted to provide a strong mechanical support. Our study shows that this approach is feasible to produce graphene with a very high surface‐area‐to‐volume ratio for possible practical applications in catalytic reactors or heat exchangers, though problems such as the inter‐diffusion between the two metal layers and defects in graphene need to be further addressed. To the best of our knowledge, this study is the first attempt to prepare graphene with high surface‐area‐to‐volume ratio by a CVD route.
Sherrell PC, Palczynski P, Sokolikova MS, et al., 2019, Large-area CVD MoS2/WS2 heterojunctions as a photoelectrocatalyst for salt water oxidation, ACS Applied Energy Materials, Vol: 2, Pages: 5877-5882, ISSN: 2574-0962
Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally <5 μm), and thus the low edge flake density, and the flakes’ atomically sharp and clean interfaces between the flakes are responsible for reducing charge carrier recombination. These results show a general approach to the scalable synthesis of high-crystal-quality low-dimensional semiconductor photoelectrodes for solar energy conversion systems. It also shows the uniqueness of the CVD synthesis process of these materials, which can lead to high quality materials without the need of any postsynthesis treatments.
Pesci FM, Brugge RH, Hekselman AKO, et al., 2018, Elucidating the role of dopants in the critical current density for dendrite formation in garnet electrolytes, Journal of Materials Chemistry A, Vol: 6, Pages: 19817-19827, ISSN: 2050-7488
Garnet-type solid electrolytes have attracted great interest in solid state battery research thanks to their high ionic conductivity at room temperature (10−3 S cm−1) and their electrochemical stability against lithium metal anodes. However, the formation of lithium dendrites following charge/discharge limits their applicability and commercialisation. Although widely investigated, no clear explanation of dendrite formation has been previously reported. In this work, we employ cubic Al- and Ga-doped Li7La3Zr2O12, which represent two of the solid electrolytes with higher technological importance, to investigate the formation and chemical composition of dendrites. For the first time, this study elucidates the role that the dopants play in determining the critical current density for dendrite formation and highlights the importance of controlling the dopant distribution in the garnet structure. We use a combination of techniques including Secondary Electron Microscopy and Secondary Ion Mass Spectrometry in order to analyse the microstructure and chemical composition of dendrites in Li7La3Zr2O12. We show that, following electrochemical cycling, Li6.55Ga0.15La3Zr2O12 systematically displays a critical current density 60% higher than Li6.55Al0.15La3Zr2O12. Chemical analysis revealed that in Li6.55Al0.15La3Zr2O12 the dendritic features are composed of a mixture of Al and Li species, whereas in Li6.55Ga0.15La3Zr2O12 they are uniquely composed of Li. We also show that only in pristine Li6.55Al0.15La3Zr2O12, the dopant segregates at the grain boundaries suggesting that local chemical inhomogeneity can have a fundamental role in the nucleation and propagation of dendrites.
Sherrell PC, Sharda K, Grotta C, et al., 2018, Thickness dependant characterization of chemically exfoliated TiS2 nanosheets, ACS Omega, Vol: 3, Pages: 8655-8662, ISSN: 2470-1343
Monolayer TiS2 is the lightest member of the transition metal dichalcogenides family with promising application in energy storage and conversion systems. Use of TiS2 has been limited by the lack of rapid characterisation of layer number via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate layer number dependent Raman modes for TiS2. 1T-TiS2 presents two characteristics Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm-1 whose intensity changes relative to the A1g mode with the thickness of TiS2 sheets. These two characteristic features of the Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL .These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over cm2 areas. This opens up pathways for a wider utilization of exofliated TiS2 towards a range of applications.
Brugge R, Hekselman A, Cavallaro A, et al., 2018, Garnet electrolytes for solid state batteries: visualization of moisture-induced chemical degradation and revealing its impact on the Li-ion dynamics, Chemistry of Materials, Vol: 30, Pages: 3704-3713, ISSN: 0897-4756
In this work, we reveal the impact of moisture-induced chemical degradation and proton–lithium exchange on the Li-ion dynamics in the bulk and the grain boundaries and at the interface with lithium metal in highly Li-conducting garnet electrolytes. A direct correlation between chemical changes as measured by depth-resolved secondary ion mass spectrometry and the change in transport properties of the electrolyte is provided. In order to probe the intrinsic effect of the exchange on the lithium kinetics within the garnet structure, isolated from secondary corrosion product contributions, controlled-atmosphere processing was first used to produce proton-free Li6.55Ga0.15La3Zr2O12 (Ga0.15-LLZO), followed by degradation steps in a H2O bath at 100 °C, leading to the removal of LiOH secondary phases at the surface. The proton-exchanged region was analyzed by focused ion beam secondary ion mass spectrometry (FIB-SIMS) and found to extend as far as 1.35 μm into the Ga0.15-LLZO garnet pellet after 30 min in H2O. Impedance analysis in symmetrical cells with Li metal electrodes indicated a greater reactivity in grain boundaries than in grains and a significantly detrimental effect on the Li transfer kinetics in the Li metal/garnet interface correlated to a 3-fold decrease in the Li mobility in the protonated garnet. This result indicates that the deterioration of Li charge transfer and diffusion kinetics in proton-containing garnet electrolytes have fundamental implications for the optimization and integration of these systems in commercial battery devices.
Pesci FM, Sokolikova MS, Grotta C, et al., 2017, MoS2/WS2 heterojunction for photoelectrochemical water oxidation, ACS Catalysis, Vol: 7, Pages: 4990-4998, ISSN: 2155-5435
The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS2 and WS2 can oxidize water to O2 under incident light. Thin films of solution-processed MoS2 and WS2 nanosheets display n-type positive photocurrent densities of 0.45 mA cm–2 and O2 evolution under simulated solar irradiation. WS2 is significantly more efficient than MoS2; however, bulk heterojunctions (B-HJs) of MoS2 and WS2 nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS2 and WS2 nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production.
Pesci FM, 2014, Metal Oxide Semiconductors Employed as Photocatalysts during Water Splitting
Photocatalytic water splitting has attracted significant interest in recent decades as it offers a clean and environmentally friendly route for the production of hydrogen. A key challenge remains the development of systems that employ abundant, non-toxic and inexpensive materials to dissociate water efficiently using sunlight. Titanium dioxide (TiO2), tungsten trioxide (WO3) and hematite (α-Fe2O3) are among the most studied photoanodes employed during water splitting because of the position of their valence band which is suitable for oxidising water to oxygen, and their low costs. However reported efficiencies for these materials are below the reported theoretical maximum values. A good understanding of the factors that are limiting the efficiency of these photoanodes is therefore desirable if improvements in the photocatalytic activity are to be achieved. This thesis is divided in four main sections. Chapters 3 and 4 describe transient absorption spectroscopy (TAS) studies in the microsecond-second timescales carried out on WO3 photoelectrodes and TiO2 nanowires respectively. TAS has been employed to follow the charge carriers dynamics in WO3 highlighting the presence of relatively long-lived holes (30 ms), which have been described as a requirement for the water oxidation reaction to take place. The electrons also appear to be long-lived (0.1 s), and this has been proposed to be due to slow electron transport through the film. TAS measurements have also been carried out on oxygen-deficient hydrogen-treated TiO2 nanowires, highlighting a more efficient suppression of the electron/hole recombination process in comparison with conventional anatase TiO2 photoanodes. Chapter 5 describes TAS and sum frequency generation (SFG) studies on TiO2 films which are designed to investigate the surface mechanisms of water oxidation. The dependence of the hole lifetime on the pH of the electrolytes employed has been examined by TAS and substantially faster decay rates have
Pastor E, Pesci FM, Reynal A, et al., 2014, Interfacial charge separation in Cu2O/RuOx as a visible light driven CO2 reduction catalyst, Physical Chemistry Chemical Physics, ISSN: 1463-9076
We employ transient absorption spectroscopy to record the absorption spectrum of photogenerated charge carriers in Cu2O. We have found that CO2 reduction in Cu2O is limited by fast electron-hole recombination. The deposition of RuOx nanoparticles on Cu2O results in a twofold increased yield of long-lived electrons, indicating partially reduced electron-hole recombination losses. This observation correlates with an approximately sixfold increase in the yield of CO2 reduction to CO.
Pesci FM, Wang G, Klug DR, et al., 2013, Efficient suppression of electron hole recombination in oxygen-deficient hydrogen-treated TiO2 nanowires for photoelectrochemical water splitting, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 117, Pages: 25837-25844, ISSN: 1932-7447
There is an increasing level of interest in the use of black TiO2 prepared by thermal hydrogen treatments (H:TiO2) due to the potential to enhance both the photocatalytic and the light-harvesting properties of TiO2. Here, we examine oxygen-deficient H:TiO2 nanotube arrays that have previously achieved very high solar-to-hydrogen (STH) efficiencies due to incident photon-to-current efficiency (IPCE) values of >90% for photoelectrochemical water splitting at only 0.4 V vs RHE under UV illumination. Our transient absorption (TA) mechanistic study provides strong evidence that the improved electrical properties of oxygen-deficient TiO2 enables remarkably efficient spatial separation of electron–hole pairs on the submicrosecond time scale at moderate applied bias, and this coupled to effective suppression of microsecond to seconds charge carrier recombination is the primary factor behind the dramatically improved photoelectrochemical activity.
Evangelisti L, Pesci F, Caminati W, 2011, Adducts of Alcohols with Ethers: The Rotational Spectrum of Isopropanol-Dimethyl Ether, JOURNAL OF PHYSICAL CHEMISTRY A, Vol: 115, Pages: 9510-9513, ISSN: 1089-5639
Pesci FM, Cowan AJ, Alexander BD, et al., 2011, Charge Carrier Dynamics on Mesoporous WO3 during Water Splitting, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 2, Pages: 1900-1903, ISSN: 1948-7185
Cowan AJ, Pendlebury SR, Barroso M, et al., 2011, Charge carrier dynamics in metal oxide water splitting photoelectrodes, 242nd ACS National Meeting
Ottaviani P, Pesci FM, Favero LB, et al., 2008, Van der Waals potential energy surface of CH(2)ClF center dot center dot center dot Xe, CHEMICAL PHYSICS LETTERS, Vol: 466, Pages: 122-126, ISSN: 0009-2614
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