Abstract: Layered Li(M,Li)O2 (where M is a transition metal) ordered rock-salt-type structures are used in advanced metal-ion batteries as one of the best hosts for the reversible intercalation of Li ions. Besides the conventional redox reaction involving oxidation/reduction of the M cation upon Li extraction/insertion, creating oxygen-located holes because of the partial oxygen oxidation increases capacity while maintaining the oxidized oxygen species in the lattice through high covalency of the M–O bonding. Typical degradation mechanism of the Li(M,Li)O2 electrodes involves partially irreversible M cation migration toward the Li positions, resulting in gradual capacity/voltage fade. Here, using LiRhO2 as a model system (isostructural and isoelectronic to LiCoO2), for the first time, we demonstrate an intimate coupling between the oxygen redox and M cation migration. A formation of the oxidized oxygen species upon electrochemical Li extraction coincides with transformation of the layered Li1–xRhO2 structure into the γ-MnO2-type rutile–ramsdellite intergrowth LiyRh3O6 structure with rutile-like [1 × 1] channels along with bigger ramsdellite-like [2 × 1] tunnels through massive and concerted Rh migration toward the empty positions in the Li layers. The oxidized oxygen dimers with the O–O distances as short as 2.26 Å are stabilized in this structure via the local Rh–O configuration reminiscent to that in the μ-peroxo-μ-hydroxo Rh complexes. The LiyRh3O6 structure is remarkably stable upon electrochemical cycling illustrating that proper structural implementation of the oxidized oxygen species can open a pathway toward deliberate employment of the anion redox chemistry in high-capacity/high-voltage positive electrodes for metal-ion batteries. Upon chemical or electrochemical oxidation, layered LiRhO2 shows a unique structural transformation that involves both cation migration and oxidation of oxygen resulting in a stable tunnel-like rutile−ramsdellite intergrowth LiyRh3O6 structure. This structure demonstrates excellent performance with the steady and reversible capacity of ∼200 mAh/g. The stability of LiyRh3O6 is rooted in the accommodation of partially oxidized oxygen species through the formation of short O−O distances that are compatible with the connectivity of RhO6 octahedra.

Abstract: Plasma catalysis is gaining increasing interest for various environmental applications, but the crucial question is whether plasma can be created inside catalyst pores and under which conditions. In practice, various catalytic support materials are used, with various dielectric constants. We investigate here the influence of the dielectric constant on the plasma properties inside catalyst pores and in the sheath in front of the pores, for various pore sizes. The calculations are performed by a two-dimensional fluid model for an atmospheric pressure dielectric barrier discharge in helium. The electron impact ionization rate, electron temperature, electron and ion density, as well as the potential distribution and surface charge density, are analyzed for a better understanding of the discharge behavior inside catalyst pores. The results indicate that, in a 100 μm pore, the electron impact ionization in the pore, which is characteristic for the plasma generation inside the pore, is greatly enhanced for dielectric constants below 300. Smaller pore sizes only yield enhanced ionization for smaller dielectric constants, i.e., up to εr = 200, 150, and 50 for pore sizes of 50, 30, and 10 μm. Thus, the most common catalyst supports, i.e., Al2O3 and SiO2, which have dielectric constants around εr = 8−11 and 4.2, respectively, should allow more easily that microdischarges can be formed inside catalyst pores, even for smaller pore sizes. On the other hand, ferroelectric materials with dielectric constants above 300 never seem to yield plasma enhancement inside catalyst pores, not even for 100 μm pore sizes. Furthermore, it is clear that the dielectric constant of the material has a large effect on the extent of plasma enhancement inside the catalyst pores, especially in the range between εr = 4 and εr = 200. The obtained results are explained in detail based on the surface charge density at the pore walls,
and the potential distribution and electron temperature inside and above the pores. The results obtained with this model are
important for plasma catalysis, as the production plasma species in catalyst pores might affect the catalyst properties, and thus
improve the applications of plasma catalysis.

Abstract: The hydrogenation of carbon dioxide in a dielectric barrier discharge plasma is studied with a one-dimensional fluid model. The spatially averaged densities of the most important end products formed in the CO2/H2 mixture are determined as a function of the initial gas mixing ratio. CO and H2O are found to be present at the highest densities and to a lower content also CH4, C2H6, CH2O, CH3OH, O2, and some other higher hydrocarbons and oxygenates. The main underlying reaction
pathways for the conversion of the inlet gases and the formation of CO, CH4, CH2O, and CH3OH are pointed out for various gas mixing ratios. The CO2 conversion and the production of value added products is found to be quite low, also in comparison to a CO2/CH4 mixture, and this can be explained by the model.

Abstract: In recent years there is increased interest in plasma-based CO2 conversion. Several plasma setups are being investigated for this purpose, but the most commonly used ones are a dielectric barrier discharge (DBD), a microwave (MW) plasma and a gliding arc (GA) reactor. In this proceedings paper, we will show results from our experiments in a (packed bed) DBD reactor and in a vortex-flow GA reactor, as well as from our model calculations for the detailed plasma chemistry in a DBD, MW and GA, for pure CO2 as well as mixtures of CO2 with N-2, CH4 and H2O.

Abstract: Solar energy plays an important role in solving serious environmental
problems and meeting the high energy demand. However, the lack of suitable
materials hinders further progress of this technology. Here, we present the largest
inorganic solar cell material search till date using density functional theory (DFT) and
machine-learning approaches. We calculated the spectroscopic limited maximum
efficiency (SLME) using the Tran−Blaha-modified Becke−Johnson potential for 5097
nonmetallic materials and identified 1997 candidates with an SLME higher than 10%,
including 934 candidates with a suitable convex-hull stability and an effective carrier
mass. Screening for two-dimensional-layered cases, we found 58 potential materials
and performed G0W0 calculations on a subset to estimate the prediction uncertainty. As the above DFT methods are still computationally expensive, we developed a high accuracy machine-learning model to prescreen efficient materials and applied it to over a million materials. Our results provide a general framework and universal strategy for the design of high-efficiency solar
cell materials. The data and tools are publicly distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html, https://www.
ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/, and https://github.com/usnistgov/jarvis.

Abstract: We performed computer simulations to investigate the effect of oxidation on the extracellular cystine (CYC) uptake by the xC− antiporter. The latter is important for killing of cancer cells. Specifically, applying molecular dynamics (MD) simulations we studied the transport of CYC across xCT, i.e., the light subunit of the xC− antiporter, in charge of bidirectional transport of CYC and glutamate. We considered the outward facing (OF) configuration of xCT, and to study the effect of oxidation, we modified the Cys327 residue, located in the vicinity of the extracellular milieu, to cysteic acid (CYO327). Our computational results showed that oxidation of Cys327 results in a free energy barrier for CYC translocation, thereby blocking the access of CYC to the substrate binding site of the OF system. The formation of the energy barrier was found to be due to the conformational changes in the channel. Analysis of the MD trajectories revealed that the reorganization of the side chains of the Tyr244 and CYO327 residues play a critical role in the OF channel blocking. Indeed, the calculated distance between Tyr244 and either Cys327 or CYO327 showed a narrowing of the channel after oxidation. The obtained free energy barrier for CYC translocation was found to be 33.9kJmol−1, indicating that oxidation of Cys327, by e.g., cold atmospheric plasma, is more effective in inhibiting the xC− antiporter than in the mutation of this amino acid to Ala (yielding a barrier of 32.4kJmol−1). The inhibition of the xC− antiporter may lead to Cys starvation in some cancer cells, eventually resulting in cancer cell death.

Abstract: Reaction of the n = 1 Ruddlesden-Popper oxide LaSr3CoRuO8 with CaH2 yields the oxyhydride phase LaSr3CoRuO4H4 via topochemical anion-exchange. Close inspection of X-ray and neutron powder diffraction data in combination with HAADF-STEM images reveals that nanoparticles of SrO are exsolved from the system during the reaction, with the change in cation stoichiometry accommodated by the inclusion of n > 1 (Co/Ru)nOn+1H2n ‘perovskite’ layers into the Ruddlesden-Popper stacking sequence. This novel pseudo-topochemical process offers a new route for the formation of n > 1 Ruddlesden-Popper structured materials. Magnetization data are consistent with a LaSr3Co1+Ru2+O4H4 (Co1+, d8, S = 1; Ru2+, d6, S = 0) oxidation/spin state combination. Neutron diffraction and μ+SR data show no evidence for long-range magnetic order down to 2 K, suggesting the diamagnetic Ru2+ centers impede the Co-Co magnetic exchange interactions.

Abstract: Plasma is gaining increasing interest for N2 fixation, being a flexible, electricity-driven alternative for the current conventional fossil fuel-based N2 fixation processes. As the vibrational-induced dissociation of N2 is found to be an energy-efficient pathway to acquire atomic N for the fixation processes, plasmas that are in vibrational nonequilibrium seem promising for this application. However, an important challenge in using nonequilibrium plasmas lies in preventing vibrational−translational (VT) relaxation processes, in which vibrational energy crucial for N2 dissociation is lost to gas heating. We present here both experimental and modeling results for the vibrational and gas temperature in a microsecond-pulsed microwave (MW) N2 plasma, showing how power pulsing can suppress this unfavorable VT relaxation and achieve a maximal vibrational nonequilibrium. By means of our kinetic model, we demonstrate that pulsed plasmas take advantage of the long time scale on which VT processes occur, yielding a very pronounced nonequilibrium over the whole N2 vibrational ladder. Additionally, the effect of pulse parameters like the pulse frequency and pulse width are investigated, demonstrating that the advantage of pulsing to inhibit VT relaxation diminishes for high pulse frequencies (around 7000 kHz) and long power pulses (above 400 μs). Nevertheless, all regimes studied here demonstrate a clear vibrational nonequilibrium while only requiring a limited power-on time, and thus, we may conclude that a pulsed plasma seems very interesting for energyefficient vibrational excitation.

Abstract: Nitrogen oxyanions and oxyacids are important agents in atmospheric chemistry and medical biology. Although their chemical behavior in solution is relatively well understood, they may behave very differently at the water/air interface of atmospheric aerosols or at the membrane/water interface of cells. Here, we developed a fully classical model for molecular dynamics simulations of NO3−, NO2−, HNO3, and HNO2 in the framework of the GROMOS 53A6 and 54A7 force field versions. The model successfully accounted for the poorly structured solvation shell and ion pairing tendency of NO3−. Accurate pure-liquid properties and hydration free energies were obtained for the oxyacids. Simulations at the water/air interface showed a local enrichment of HNO3 and depletion of NO3−. The effect was discussed in light of earlier spectroscopic data and ab initio calculations, suggesting that HNO3 behaves as a weaker acid at the surface of water. Our model will hopefully allow for efficient and accurate simulations of nitrogen oxyanions and oxyacids in solution and at microheterogeneous interface environments.

Abstract: In this work, we use density functional theory calculations to study the combined effect of external electric fields, surface morphology, and surface charge on CO2 activation over Cu(111), Cu(211), Cu(110), and Cu(001) surfaces. We observe that the binding energy of the CO2 molecule on Cu surfaces increases significantly upon increasing the applied electric field strength. In addition, rougher surfaces respond more effectively to the presence of the external electric field toward facilitating the formation of a carbonate-like CO2 structure and the transformation of the most stable adsorption mode from physisorption to chemisorption. The presence of surface charges further strengthens the electric field effect and consequently causes an improved bending of the CO2 molecule and C−O bond length elongation. On the other hand, a net charge in the absence of an externally applied electric field shows only a marginal effect on CO2 binding. The chemisorbed CO2 is more stable and further activated when the effects of an external electric field, rough surface, and surface charge are combined. These results can help to elucidate the underlying factors that control CO2 activation in heterogeneous and plasma catalysis, as well as in electrochemical processes.

Abstract: Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag–Au alloy, Ag@Au core–shell, and Au@Ag core–shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination of optical constants of Au–Ag alloys reported in the literature. Although both Au–Ag core–shell and alloy nanoparticles exhibit an intermediary optical response between that of pure Ag and Au nanoparticles, there are noticeable differences in the spectral characteristics. Also, the effect of the bimetallic constitution in anisotropic nanoparticles is starkly different from that in spherical nanoparticles due to the absence of Au interband transitions in the former case. In general, the improved chemical stability of Ag nanoparticles by incorporation of Au comes with a cost of reduction in plasmonic enhancement, also applicable to anisotropic nanoparticles with a weaker effect. A photothermal heat transfer study confirms that increased absorption by the incorporation of Au in spherical Ag nanoparticles also results in an increased steady-state temperature. On the other hand, anisotropic nanoparticles are inherently better absorbers and hence better photothermal sources, and their photothermal properties are apparently not strongly affected by the incorporation of one metal in the other. This study of the optical/spectral and photothermal characteristics of bimetallic Au–Ag alloy versus core–shell nanoparticles provides detailed physical insight for development of new taylor-made plasmonic nanostructures.

Abstract: The combination of catalysis with non-thermal plasma is a promising alternative to thermal catalysis. A dielectric-barrier discharge reactor was used to study plasma-catalytic dry reforming of methane at ambient pressure and temperature and a fixed plasma power of 45 W. The effect of different catalytic packing materials was evaluated in terms of conversion, product selectivity, and energy efficiency. The conversion of CO2 (~22%) and CH4 (~33%) were found to be similar in plasma-only and when introducing packing materials in plasma. The main reason is the shorter residence time of the gas due to packing geometry, when compared at identical flow rates. H2, CO, C2-C4 hydrocarbons, and oxygenates were identified in the product gas. High selectivity towards H2 and CO were found for all catalysts and plasma-only, with a H2/CO molar ratio of ~0.9. The lowest syngas selectivity was obtained with Cu/Al2O3 (~66%), which instead, had the highest alcohol selectivity (~3.6%).

Abstract: This paper gives an overview of the modeling work developed in our group in the last 25 years for various plasmas used in analytical spectrochemistry, i.e., glow discharges (GDs), inductively coupled plasmas (ICPs), and laser ablation (LA) for sample introduction in the ICP and for laser-induced breakdown spectroscopy (LIBS). The modeling approaches are briefly presented, which are different for each case, and some characteristic results are illustrated. These plasmas are used not only in analytical chemistry but also in other applications, and the insights obtained in these other fields were quite helpful for us to develop models for the analytical plasmas. Likewise, there is now a huge interest in plasma–liquid interaction, atmospheric pressure glow discharges (APGDs), and dielectric barrier discharges (DBDs) for environmental, medical, and materials applications of plasmas. The insights obtained in these fields are also very relevant for ambient desorption/ionization sources and for liquid sampling, which are nowadays very popular in analytical chemistry, and they could be very helpful in developing models for these sources as well.

Abstract: Ammonia is one of the most produced chemicals, mainly synthesized from fossil fuels for fertilizer applications. Furthermore, ammonia may be one of the energy carriers of the future, when it is produced from renewable electricity. This has spurred research on alternative technologies for green ammonia production. Research on plasma-driven ammonia synthesis has recently gained traction in academic literature. In the current review, we summarize the literature on plasma-driven ammonia synthesis. We distinguish between mechanisms for ammonia synthesis in the presence of a plasma, with and without a catalyst, for different plasma conditions. Strategies for catalyst design are discussed, as well as the current understanding regarding the potential plasma-catalyst synergies as function of the plasma conditions and their implications on energy efficiency. Finally, we discuss the limitations in currently reported models and experiments, as an outlook for research opportunities for further unravelling the complexities of plasma-catalytic ammonia synthesis, in order to bridge the gap between the currently reported models and experimental results.

Abstract: Biological membranes are under constant attack of free radicals, which may lead to lipid nitro-oxidation, pro­ ducing a complex mixture of nitro-oxidized lipids that are responsible for structural and dynamic changes on the membrane. Despite the latter, nitro-oxidized lipids are also associated with several inflammatory and neuro­ degenerative diseases, the underlying mechanisms of which remain elusive. We perform atomistic molecular dynamics simulations using several isomers of nitro-oxidized lipids to study their effect on the structure and permeability of the membrane, as well as the interaction between the mixture of these products in the phos­pholipid membrane environment. Our results show that the stereo- and positional isomers have a stronger effect on the properties of the membrane composed of oxidized lipids compared to that containing nitrated lipids. Nevertheless, nitrated lipids lead to three-fold increase in water permeability compared to oxidized lipids. In addition, we show that in a membrane consisting of combined nitro-oxidized lipid products, the presence of oxidized lipids protects the membrane from transient pores. Is well stablished that plasma application and photodynamic therapy produces a number of oxidative species used to kill cancer cells, through membrane damage induced by nitro-oxidative stress. This study is important to elucidate the mechanisms and the molecular level properties involving the reactive species produced during that cancer therapies.