Development of Electrode-Supported Proton Conducting Solid Oxide Cells and their Evaluation as Electrochemical Hydrogen Pumps.

Protonic ceramic solid oxide cells (P-SOCs) have gained widespread attention due to their potential for operation in the temperature range of 300-500 °C, which is not only beneficial in terms of material stability but also offers unique possibilities from a thermodynamic point of view to realize a series of reactions. For instance, they are ideal for the production of synthetic fuels by hydrogenation of carbon dioxide and nitrogen, upgradation of hydrocarbons, or dehydrogenation reactions. However, the development of P-SOC is quite challenging because it requires a multifront optimization in terms of material synthesis and fabrication procedures. Herein, we report in detail a method to overcome various fabrication challenges for the development of efficient and robust electrode-supported P-SOCs (Ni-BCZY/BCZY/Ni-BCZY) based on a BaCe0.2Zr0.7Y0.1O3-δ (BCZY271) electrolyte. We examined the effect of pore formers on the porosity of the Ni-BCZY support electrode, various electrolyte deposition techniques (spray, spin, and vacuum-assisted), and thermal treatments for developing robust and flat half-cells. Half-cells containing a thin (10-12 µm) pinhole-free electrolyte layer were completed by a screen-printed Ni-BCZY electrode and evaluated as an electrochemical hydrogen pump to access the functionality. The P-SOCs are found to show a current density ranging from 150 to 525 mA cm-2 at 1 V over an operating temperature range of 350-450 °C. The faradaic efficiency of the P-SOCs as well as their stability were also evaluated.

The Chemical Origins of Plasma Contraction and Thermalization in CO2 Microwave Discharges.

Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with the first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from ∼7 mm diameter at 100 mbar to ∼2 mm at 400 mbar. We find that, simultaneously, the initial nonequilibrium between ∼2 eV electron and ∼0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modeling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors.

Revisiting spontaneous Raman scattering for direct oxygen atom quantification.

In this Letter, the counterintuitive and largely unknown Raman activity of oxygen atoms is evaluated for its capacity to determine absolute densities in gases with significant O-density. The study involves ${\rm CO}_2$ microwave plasma to generate a self-calibrating mixture and establish accurate cross sections for the $^3{\!P_2}{\leftrightarrow ^3}{\!P_1}$ and $^3{\!P_2}{\leftrightarrow ^3}{\!P_0}$ transitions. The approach requires conservation of stoichiometry, confirmed within experimental uncertainty by a 1D fluid model. The measurements yield ${\sigma _{J = 2 \to 1}} = 5.27 \pm _{{\rm sys}:0.53}^{{\rm rand}:0.17} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$ and ${\sigma _{J = 2 \to 0}} = 2.11 \pm _{{\rm sys}:0.21}^{{\rm rand}:0.06} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$, and the detection limit is estimated to be $1 \times {10^{15}}\;{{\rm cm}^{- 3}}$ for systems without other scattering species.

High and intermediate temperature sodium-sulfur batteries for energy storage: development, challenges and perspectives.

In view of the burgeoning demand for energy storage stemming largely from the growing renewable energy sector, the prospects of high (>300 °C), intermediate (100-200 °C) and room temperature (25-60 °C) battery systems are encouraging. Metal sulfur batteries are an attractive choice since the sulfur cathode is abundant and offers an extremely high theoretical capacity of 1672 mA h g-1 upon complete discharge. Sodium also has high natural abundance and a respectable electrochemical reduction potential (-2.71 V vs. standard hydrogen electrode). Combining these two abundant elements as raw materials in an energy storage context leads to the sodium-sulfur battery (NaS). This review focuses solely on the progress, prospects and challenges of the high and intermediate temperature NaS secondary batteries (HT and IT NaS) as a whole. The already established HT NaS can be further improved in terms of energy density and safety record. The IT NaS takes advantage of the lower operating temperature to lower manufacturing and potentially operating costs whilst creating a safer environment. A thorough technical discussion on the building blocks of these two battery systems is discussed here, including electrolyte, separators, cell configuration, electrochemical reactions that take place under the different operating conditions and ways to monitor and comprehend the physicochemical and electrochemical processes under these temperatures. Furthermore, a brief summary of the work conducted on the room temperature (RT) NaS system is given seeking to couple the knowledge in this field with the one at elevated temperatures. Finally, future perspectives are discussed along with ways to effectively handle the technical challenges presented for this electrochemical energy storage system.

How the alternating degeneracy in rotational Raman spectra of CO2 and C2H2 reveals the vibrational temperature.

The contribution of higher vibrational levels to the rotational spectrum of linear polyatomic molecules with a center of symmetry (CO2 and C2H2) is assessed. An apparent nuclear degeneracy is analytically formulated by vibrational averaging and compared to numerical averaging over vibrational levels. It enables inferring the vibrational temperature of the bending and asymmetric stretching modes from the ratio of even to odd peaks in the rotational Raman spectrum. The contribution from higher vibrational levels is already observable at room temperature as g˜e/o=0.96/0.04 for CO2 and g˜e/o=1.16/2.84 for C2H2. The use of the apparent degeneracy to account for higher vibrational levels is demonstrated on spectra measured for a CO2 microwave plasma in the temperature range of 300-3500 K, and shown to be valid up to 1500 K.

Atomistic simulations of graphite etching at realistic time scales.

Hydrogen-graphite interactions are relevant to a wide variety of applications, ranging from astrophysics to fusion devices and nano-electronics. In order to shed light on these interactions, atomistic simulation using Molecular Dynamics (MD) has been shown to be an invaluable tool. It suffers, however, from severe time-scale limitations. In this work we apply the recently developed Collective Variable-Driven Hyperdynamics (CVHD) method to hydrogen etching of graphite for varying inter-impact times up to a realistic value of 1 ms, which corresponds to a flux of ∼1020 m-2 s-1. The results show that the erosion yield, hydrogen surface coverage and species distribution are significantly affected by the time between impacts. This can be explained by the higher probability of C-C bond breaking due to the prolonged exposure to thermal stress and the subsequent transition from ion- to thermal-induced etching. This latter regime of thermal-induced etching - chemical erosion - is here accessed for the first time using atomistic simulations. In conclusion, this study demonstrates that accounting for long time-scales significantly affects ion bombardment simulations and should not be neglected in a wide range of conditions, in contrast to what is typically assumed.

Oscillatory vapour shielding of liquid metal walls in nuclear fusion devices.

Providing an efficacious plasma facing surface between the extreme plasma heat exhaust and the structural materials of nuclear fusion devices is a major challenge on the road to electricity production by fusion power plants. The performance of solid plasma facing surfaces may become critically reduced over time due to progressing damage accumulation. Liquid metals, however, are now gaining interest in solving the challenge of extreme heat flux hitting the reactor walls. A key advantage of liquid metals is the use of vapour shielding to reduce the plasma exhaust. Here we demonstrate that this phenomenon is oscillatory by nature. The dynamics of a Sn vapour cloud are investigated by exposing liquid Sn targets to H and He plasmas at heat fluxes greater than 5 MW m-2. The observations indicate the presence of a dynamic equilibrium between the plasma and liquid target ruled by recombinatory processes in the plasma, leading to an approximately stable surface temperature.Vapour shielding is one of the interesting mechanisms for reducing the heat load to plasma facing components in fusion reactors. Here the authors report on the observation of a dynamic equilibrium between the plasma and the divertor liquid Sn surface leading to an overall stable surface temperature.

Self-Regulated Plasma Heat Flux Mitigation Due to Liquid Sn Vapor Shielding.

A steady-state high-flux H or He plasma beam was balanced against the pressure of a Sn vapor cloud for the first time, resulting in a self-regulated heat flux intensity near the liquid surface. A temperature response of the liquid surface characterized by a decoupling from the received heating power and significant cooling of the plasma in the neutral Sn cloud were observed. The plasma heat flux impinging on the target was found to be mitigated, as heat was partially dissipated by volumetric processes in the vapor cloud rather than wholly by surface effects. These results motivate further exploration of liquid metal solutions to the critical challenge of heat and particle flux handling in fusion power plants.

Taming microwave plasma to beat thermodynamics in CO2 dissociation.

The strong non-equilibrium conditions provided by the plasma phase offer the opportunity to beat traditional thermal process energy efficiencies via preferential excitation of molecular vibrations. Simple molecular physics considerations are presented to explain potential dissociation pathways in plasma and their effect on energy efficiency. A common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (exceeding 10(4) K) and conversion degrees (up to 30%), respectively. The results are interpreted on a basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry to dominate. The highest observed energy efficiencies of 45% indicate that non-equilibrium dynamics had been at play. A novel approach involving additives of low ionization potential to tailor the electron energies to the vibrational excitation regime is proposed.

Note: Rotational Raman scattering on CO2 plasma using a volume Bragg grating as a notch filter.

We present a novel approach for filtering Rayleigh scattering and stray light from Raman scattering in a gas discharge, using a volume Bragg grating as a notch filter. For low frequency rotational Raman contributions, it is essential to filter out Rayleigh scattering and stray light at the laser wavelength to be able to measure an undisturbed Raman spectrum. Using the Bragg grating, having an optical density of 3.1 at the central wavelength of 532 nm and a full width at half maximum of 7 cm(-1), we were able to measure a nearly full rotational CO2 spectrum (1.56 cm(-1) peak-to-peak separation). The rotational temperature in a CO2 discharge was determined with an accuracy of 2%.

Gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals with short- and long-chain alkynes.

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of ∼58%, which is close to the theoretical maximum of ∼55% and ∼60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites.

Population inversion in a magnetized hydrogen plasma expansion as a consequence of the molecular mutual neutralization process.

A weakly magnetized expanding hydrogen plasma, created by a cascaded arc, was investigated using optical emission spectroscopy. The emission of the expanding plasma is dominated by H{α} emission in the first part of the plasma expansion, after which a sharp transition to a blue afterglow is observed. The position of this sharp transition along the expansion axis depends on the magnetic field strength. The blue afterglow emission is associated with population inversion of the electronically excited atomic hydrogen states n=4-6 with respect to n=3. By comparing the measured densities with the densities using an atomic collisional radiative model, we conclude that atomic recombination processes cannot account for the large population densities observed. Therefore, molecular processes must be important for the formation of excited states and for the occurrence of population inversion. This is further corroborated at the transition from red to blue, where a hollow profile of the excited states n=4-6 in the radial direction is observed. This hollow profile is explained by the molecular mutual neutralization process of H2+ with H⁻, which has a maximum production for excited atomic hydrogen 1-2 cm outside the plasma center.

A capacitive probe with shaped probe bias for ion flux measurements in depositing plasmas.

The application of a pulse shaped biasing method implemented to a capacitive probe is described. This approach delivers an accurate and simple way to determine ion fluxes in diverse plasma mixtures. To prove the reliability of the method, the ion probe was used in a different configuration, namely, a planar Langmuir probe. In this configuration, the ion current was directly determined from the I-V characteristic and compared with the ion current measured with the pulse shaped ion probe. The results from both measurements are in excellent agreement. It is demonstrated that the capacitive probe is able to perform spatially resolved ion flux measurements under high deposition rate conditions (2-20 nm/s) in a remote expanding thermal plasma in Ar/NH(3)/SiH(4) mixture.

Influence of rarefaction on the flow dynamics of a stationary supersonic hot-gas expansion.

The gas dynamics of a stationary hot-gas jet supersonically expanding into a low pressure environment is studied through numerical simulations. A hybrid coupled continuum-molecular approach is used to model the flow field. Due to the low pressure and high thermodynamic gradients, continuum mechanics results are doubtful, while, because of its excessive time expenses, a full molecular method is not feasible. The results of the hybrid coupled continuum-molecular approach proposed have been successfully validated against experimental data by R. Engeln [Plasma Sources Sci. Technol. 10, 595 (2001)] obtained by means of laser induced fluorescence. Two main questions are addressed: the necessity of applying a molecular approach where rarefaction effects are present in order to correctly model the flow and the demonstration of an invasion of the supersonic part of the flow by background particles. A comparison between the hybrid method and full continuum simulations demonstrates the inadequacy of the latter, due to the influence of rarefaction effects on both velocity and temperature fields. An analysis of the particle velocity distribution in the expansion-shock region shows clear departure from thermodynamic equilibrium and confirms the invasion of the supersonic part of the flow by background particles. A study made through particles and collisions tracking in the supersonic region further proves the presence of background particles in this region and explains how they cause thermodynamic nonequilibrium by colliding and interacting with the local particles.

Conformal coverage of poly(3,4-ethylenedioxythiophene) films with tunable nanoporosity via oxidative chemical vapor deposition.

Novel nanoporous poly(3,4-ethylenedioxythiophene) (PEDOT) films with basalt-like surface morphology are successfully obtained via a one-step, vapor phase process of oxidative chemical vapor deposition (oCVD) by introducing a new oxidant, CuCl(2). The substrate temperature of the oCVD process is a crucial process parameter for controlling electrical conductivity and conjugation length. Moreover, the surface morphology is also systemically tunable through variations in substrate temperature, a unique advantage of the oCVD process. By increasing the substrate temperature, the surface morphology becomes more porous, with the textured structure on the nanometer scale. The size of nanopores and fibrils appears uniformly over 25 mm x 25 mm areas on the Si wafer substrates. Conformal coverage of PEDOT films grown with the CuCl(2) oxidant (C-PEDOT) is observed on both standard trench structures with high aspect ratio and fragile surfaces with complex topology, such as paper, results which are extremely difficult to achieve with liquid phase based processes. The tunable nanoporosity and its conformal coverage on various complex geometries are highly desirable for many device applications requiring controlled, high interfacial area, such as supercapacitors, Li ion battery electrodes, and sensors. For example, a highly hydrophilic surface with the static water contact angle down to less than 10 degrees is obtained solely by changing surface morphology. By applying fluorinated polymer film onto the nanoporous C-PEDOT via initiative chemical vapor deposition (iCVD), the C-PEDOT surface also shows the contact angle higher than 150 degrees . The hierarchical porous structure of fluorinated polymer coated C-PEDOT on a paper mat shows superhydrophobicity and oil repellency.

Production mechanisms of NH and NH2 radicals in N2-H2 plasmas.

We measured the densities of NH and NH(2) radicals by cavity ring-down spectroscopy in N(2)-H(2) plasmas expanding from a remote thermal plasma source and in N(2) plasmas to which H(2) was added in the background. The NH radical was observed via transitions in the (0,0), (1,1), and (2,2) vibrational bands of the A(3)Pi <-- X(3)Sigma- electronic transition and the NH(2) radical via transitions in the (0,9,0) <-- (0,0,0) band of the A(2)A(1) <-- X(2)B(1) electronic transition. The measurements revealed typical densities of 5 x 10(18) m(-3) for the NH radical in both plasmas and up to 7 x 10(18) m(-3) for the NH(2) radical when N(2) and H(2) are both fed through the plasma source. In N(2) plasma with H(2) injected in the background, no NH(2) was detected, indicating that the density is below our detection limit of 3 x 1016 m-3. The error in the measured densities is estimated to be around 20%. From the trends of the NH(x) radicals as a function of the relative H(2) flow to the total N(2) and H(2) flow at several positions in the expanding plasma beam, the key reactions for the formation of NH and NH(2) have been determined. The NH radicals are mainly produced via the reaction of N atoms emitted by the plasma source with H(2) molecules with a minor contribution from the reaction of N+ with H(2). The NH(2) radicals are formed by reactions of NH(3) molecules, produced at the walls of the plasma reactor, and H atoms emitted by the plasma source. The NH radicals can also be produced by H abstraction of NH(2) radicals. The flux densities of the NH(x) radicals with respect to the atomic radicals are appreciable in the first part of the expansion. Further downstream the NH(x) radicals are dissociated, and their densities become smaller than those of the atomic radicals. It is concluded that the NH(x) radicals play an important role as precursors for the N and H atoms, which are key to the surface production of N(2), H(2), and NH(3) molecules.

Quasi-ice monolayer on atomically smooth amorphous SiO2 at room temperature observed with a high-finesse optical resonator.

The structure of an H(2)O monolayer bound to atomically smooth hydroxylated amorphous silica is probed under ambient conditions by near-infrared evanescent-wave cavity ring-down absorption spectroscopy. Employing a miniature monolithic optical resonator, we find sharp (approximately 10 cm(-1)) and polarized (>10:1) vibration-combination bands for surface OH and adsorbed H(2)O, which reveal ordered species in distinct local environments. Indicating first-monolayer uniqueness, the absorption bands for adsorbed H(2)O show intensity saturation and line narrowing with completion of one monolayer. Formation of the ordered H(2)O monolayer likely arises from H bonding to a quasicrystalline surface OH network.

Detailed TIMS study of Ar/C(2)H(2) expanding thermal plasma: identification of a-C:H film growth precursors.

Acetylene chemistry is studied by means of threshold ionization mass spectrometry (TIMS) in remote Ar/C(2)H(2) expanding thermal plasma to identify the growth precursors of hydrogenated amorphous carbon (a-C:H) films. More than 20 hydrocarbon species are measured, enabling a comprehensive study of acetylene chemistry in the plasma environment. It is shown that the plasma composition is controlled by the initial ratio between the acetylene flow into the reactor and argon ion and electron fluence emanating from the remote plasma source. Complete decomposition of acetylene to C, CH, CH(2), C(2), and C(2)H radicals is achieved in subsequent charge transfer and dissociative recombination reactions under low acetylene flow conditions. The formation of soft polymer-like a-C:H films can be attributed to C, C(2), and also partially to CH and C(2)H deposition. At acetylene flows higher than argon ion and electron fluence, reactions of C, CH, C(2), and C(2)H radicals with acetylene lead to the formation of various hydrocarbon species, whose behavior is dependent on whether the number of carbon atoms is even or odd. The detected resonantly stabilized C(3), C(3)H, and probably also C(5) and C(5)H radicals are unreactive with acetylene in the gas phase and are, therefore, abundantly present close to the substrate. The C(3) radical has among them the highest density, and it is identified as the significant growth precursor of Ar/C(2)H(2) expanding thermal plasma deposited hard a-C:H films.