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Abstract. Isoprene-derived secondary organic aerosol (iSOA) is a significantcontributor to organic carbon (OC) in some forested regions, such astropical rainforests and the Southeastern US. However, its contribution toorganic aerosol in urban areas that have high levels of anthropogenicpollutants is poorly understood. In this study, we examined the formation ofanthropogenically influenced iSOA during summer in Beijing, China. Localisoprene emissions and high levels of anthropogenic pollutants, inparticular NOx and particulate SO42-, led to the formation ofiSOA under both high- and low-NO oxidation conditions, with significantheterogeneous transformations of isoprene-derived oxidation products toparticulate organosulfates (OSs) and nitrooxy-organosulfates (NOSs).Ultra-high-performance liquid chromatography coupled to high-resolution massspectrometry was combined with a rapid automated data processing techniqueto quantify 31 proposed iSOA tracers in offline PM2.5 filterextracts. The co-elution of the inorganic ions in the extracts caused matrixeffects that impacted two authentic standards differently. The averageconcentration of iSOA OSs and NOSs was 82.5 ng m−3, which was around 3 timeshigher than the observed concentrations of their oxygenated precursors(2-methyltetrols and 2-methylglyceric acid). OS formation was dependant onboth photochemistry and the sulfate available for reactive uptake, as shown by astrong correlation with the product of ozone (O3) and particulatesulfate (SO42-). A greater proportion of high-NO OS products wereobserved in Beijing compared with previous studies in less pollutedenvironments. The iSOA-derived OSs and NOSs represented 0.62 %of the oxidized organic aerosol measured by aerosol mass spectrometry on average, butthis increased to ∼3 % on certain days. These resultsindicate for the first time that iSOA formation in urban Beijing is stronglycontrolled by anthropogenic emissions and results in extensive conversion toOS products from heterogenous reactions. 

Cation‐disordered rocksalt (DRX) cathodes have recently emerged as a promising class of cobalt‐free, high‐capacity cathodes for lithium‐ion batteries. To facilitate their commercialization, the development of scalable synthesis techniques providing control over composition and morphology is critical. To this end, a sol‐gel synthesis route to prepare Mn‐rich DRX cathodes with high capacities is presented here. Several compositions with varied Mn content and nominal F doping are successfully prepared using this technique. In‐situ X‐ray diffraction measurements demonstrate that DRX formation proceeds at moderate temperature (800 °C) through the sol‐gel route, which enables intimate mixing among reactive intermediate phases that form at lower temperatures. All synthesized compositions possess cation short‐range order, as evidenced by neutron pair distribution function and electron diffraction analysis. These DRX materials demonstrate promising electrochemical performance with reversible capacities up to 275 mAh g. Compared to the baseline oxide (Li_1.2Mn_0.4Ti_0.4O₂), the Mn‐rich compositions exhibit improved cycling stability, with some showing an increase in capacity upon cycling. Overall, this study demonstrates the feasibility of preparing high‐capacity DRX cathodes through a sol‐gel based synthesis route, which may be further optimized to provide better control over the product morphology compared to traditional synthesis methods.

 

Relating the synthesis conditions of materials to their functional performance has long been an experience‐based trial‐and‐error process. However, this methodology is not always efficient in identifying an appropriate protocol and can lead to overlooked opportunities for the performance optimization of materials through simple modifications of the synthesis process. In this work, the authors systematically track the structural evolution in the synthesis of a representative disordered rock salt (a promising next‐generation Li‐ion cathode material) at the scale of both the long‐range crystal structure and the short‐range atomic structure using various in situ and ex situ techniques, including transmission electron microscopy, X‐ray diffraction, and pair distribution function analysis. An optimization strategy is proposed for the synthesis protocol, leading to a remarkably enhanced capacity (specific energy) of 313 mAh g⁻¹(987 Wh kg⁻¹) at a low rate (20 mA g⁻¹), with a capacity of more than 140 mAh g⁻¹retained even at a very high cycling rate of 2000 mA g⁻¹. This strategy is further rationalized using ab initio calculations, and important opportunities for synthetic optimization demonstrated in this study are highlighted.

 

Fluorine substitution is a critical enabler for improving the cycle life and energy density of disordered rocksalt (DRX) Li‐ion battery cathode materials which offer prospects for high energy density cathodes, without the reliance on limited mineral resources. Due to the strong Li–F interaction, fluorine also is expected to modify the short‐range cation order in these materials which is critical for Li‐ion transport. In this work, density functional theory and Monte Carlo simulations are combined to investigate the impact of Li–F short‐range ordering on the formation of Li percolation and diffusion in DRX materials. The modeling reveals that F substitution is always beneficial at sufficiently high concentrations and can, surprisingly, even facilitate percolation in compounds without Li excess, giving them the ability to incorporate more transition metal redox capacity and thereby higher energy density. It is found that for F levels below 15%, its effect can be beneficial or disadvantageous depending on the intrinsic short‐range order in the unfluorinated oxide, while for high fluorination levels the effects are always beneficial. Using extensive simulations, a map is also presented showing the trade‐off between transition‐metal capacity, Li‐transport, and synthetic accessibility, and two of the more extreme predictions are experimentally confirmed.

 

Abstract. Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models.

This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

 

The recent discovery of Li‐excess cation‐disordered rock salt cathodes has greatly enlarged the design space of Li‐ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn³⁺–Nb⁵⁺‐based cation‐disordered oxyfluorides, Li_1.2Mn³⁺_0.6+0.5xNb⁵⁺_0.2−0.5xO_2−xF_x(x= 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition‐metal redox (Mn³⁺/Mn⁴⁺) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn³⁺‐induced Jahn–Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn‐redox capacity to the overall capacity.