Anion exchange membranes (AEMs) find widespread applications as an electrolyte and/or electrode binder in fuel cells, electrodialysis stacks, flow and metal-air batteries, and electrolyzers. AEMs ...exhibit poor stability in alkaline media; their degradation is induced by the hydroxide ion, a potent nucleophile. We have used 2D NMR techniques to investigate polymer backbone stability (as opposed to cation stability) of the AEM in alkaline media. We report the mechanism behind a peculiar, often-observed phenomenon, wherein a demonstrably stable polysulfone backbone degrades rapidly in alkaline solutions upon derivatization with alkaline stable fixed cation groups. Using COSY and heteronuclear multiple quantum correlation spectroscopy (2D NMR), we unequivocally demonstrate that the added cation group triggers degradation of the polymer backbone in alkaline via quaternary carbon hydrolysis and ether hydrolysis, leading to rapid failure. This finding challenges the existing perception that having a stable cation moiety is sufficient to yield a stable AEM and emphasizes the importance of the often ignored issue of backbone stability.
Rutile phase tantalum-modified titanium oxide (Ta0.3Ti0.7O2) was synthesized and studied using electrochemical and spectroscopic methods to evaluate its efficacy as a corrosion-resistant ...electrocatalyst support material. A 20 wt % Pt supported on Ta0.3Ti0.7O2 catalyst was prepared and compared in terms of activity and stability against a 20 wt % Pt supported on Vulcan XC-72R carbon catalyst (20% Pt/C; synthesized in-house) and a 46 wt % Pt/C commercially sourced catalyst (Tanaka KK). Catalysts 20% Pt/Ta0.3Ti0.7O2, 20% Pt/C, and 46% Pt/C possessed electrochemically active surface areas (ECSAs) of 60, 57, and 65 m2 g–1, respectively, and mass activities for the oxygen reduction reaction (at 0.9 V vs RHE) of 185, 148, and 224 mA mg–1 Pt, respectively, as evaluated in an operating polymer electrolyte fuel cell. Accelerated stability tests (ASTs) were performed on membrane electrode assemblies (MEAs) in an operating fuel cell to investigate both support and platinum catalyst stability. The loss in voltage at a current density of 0.4 Acm–2 after 10 000 support stability AST cycles was only 23 mV for 20% Pt/Ta0.3Ti0.7O2, over an order of magnitude lower than the losses observed in 20% Pt/C and 46% Pt/C (∼330 mV). Although the latter loss would correspond to catastrophic fuel cell and stack failure, the former is well within the limits of system tolerance. Post-mortem transmission electron microscopy (TEM) analyses of the electrocatalyst recovered from cycled MEAs confirmed the excellent stability of Pt nanoparticles supported on Ta0.3Ti0.7O2. The average Pt particle size increased by ∼20% in 20% Pt/Ta0.3Ti0.7O2, as compared with a doubling in size in the case of 20% Pt/C and a near tripling in size in 46% Pt/C. The existence of strong metal–support interactions in 20% Pt/Ta0.3Ti0.7O2 was ascertained from the X-ray absorption near edge structure analysis. The number of unfilled d states in 20% Pt/Ta0.3Ti0.7O2 was found to be ∼1.47, which was lower than the value of ∼1.60 found in both the carbon-supported Pt catalysts. The decrease in the number of unfilled d states confirmed electron donation from the Ta0.3Ti0.7O2 support to the Pt atoms, resulting in an increased electron density on Pt. This interaction enhanced both electrocatalytic activity and catalyst stability, as evidenced by the results above.
Although we can increasingly measure transcription, chromatin, methylation, and other aspects of molecular biology at single-cell resolution, most assays survey only one aspect of cellular biology. ...Here we describe sci-CAR, a combinatorial indexing-based coassay that jointly profiles chromatin accessibility and mRNA (CAR) in each of thousands of single cells. As a proof of concept, we apply sci-CAR to 4825 cells, including a time series of dexamethasone treatment, as well as to 11,296 cells from the adult mouse kidney. With the resulting data, we compare the pseudotemporal dynamics of chromatin accessibility and gene expression, reconstruct the chromatin accessibility profiles of cell types defined by RNA profiles, and link cis-regulatory sites to their target genes on the basis of the covariance of chromatin accessibility and transcription across large numbers of single cells.
Fuel and oxygen harvesting from Martian regolithic brine Gayen, Pralay; Sankarasubramanian, Shrihari; Ramani, Vijay K.
Proceedings of the National Academy of Sciences - PNAS,
12/2020, Letnik:
117, Številka:
50
Journal Article
Recenzirano
Odprti dostop
NASA’s current mandate is to land humans on Mars by 2033. Here, we demonstrate an approach to produce ultrapure H₂ and O₂ from liquid-phase Martian regolithic brine at ∼−36 °C. Utilizing a Pb₂Ru₂O7–δ ...pyrochlore O₂-evolution electrocatalyst and a Pt/C H₂-evolution electrocatalyst, we demonstrate a brine electrolyzer with >25× the O₂ production rate of the Mars Oxygen In Situ Resource Utilization Experiment (MOXIE) from NASA’s Mars 2020 mission for the same input power under Martian terrestrial conditions. Given the Phoenix lander’s observation of an active water cycle on Mars and the extensive presence of perchlorate salts that depress water’s freezing point to ∼−60 °C, our approach provides a unique pathway to life-support and fuel production for future human missions to Mars.
Developing highly active oxygen evolution and reduction reaction (OER/ORR) bifunctional electrocatalysts is key to multiple technologies, including regenerative fuel cells and metal-air batteries. To ...this end, we have investigated structure–activity relationships in Pb2Ru2O7–x having pyrochlore structure by tuning the structural oxygen vacancy (Ovac) and metal oxidation states. Increase in Ovac with temperature boosts the ORR activity by facilitating molecular oxygen dissociation via decrease in work function. Ovac formation is accompanied by lowering of the Ru(V)/Ru(IV) ratio due to charge-compensation which leads to decreased OER activity. Air-annealing of Pb2Ru2O7–x accelerates the formation of Ovac in comparison to Ar-annealing since atmospheric oxygen facilitates the reduction and phase-segregation of Ru from Pb2Ru2O7–x as RuO2. A maximum bifunctionality index and specific bifunctionality index of 0.69 V and 274.0 μA/cm2 BET, respectively, are observed for pristine Pb2Ru2O7–x . However, the activity is skewed toward OER for pristine Pb2Ru2O7–x , creating an asymmetric bifunctional property which is not desirable for practical applications. To reduce the asymmetric behavior, pristine and air-annealed Pb2Ru2O7–x samples at 700 °C are physically mixed which yields a higher symmetric OER/ORR activity (|Δi OER(η = 0.25 V)‑ORR(η = −0.45 V) specific|: pristine = 0.25 mA/cm2 BET, Air-700 °C = 0.20 mA/cm2 BET, physical mixture = 0.037 mA/cm2 BET). The inverse OER/ORR relationship in Pb2Ru2O7–x is attributed to the presence of an optimal ratio of 0.75 for Ru(V)/Ru(IV) and Ovac/Olattice, which provides symmetric bifunctional activity essential in electrochemical devices. An increase in Ru(V)/Ru(IV) ratio in pristine Pb2Ru2O7–x with no detectable Ru dissolution in the electrolyte observed subsequent to a 5-h OER hold-test, confirming high stability.
Redox flow batteries (RFBs) hold great potential for large‐scale, extended‐duration stationary energy storage. Here, a novel computationally cost‐effective hydraulic‐electrical analogous model (HEAM) ...for fluid flow in RFBs is developed. The HEAM demonstrated that lowering the electrode compression and enhancing the channel area lowers the pump power loss independent of the flow fields and electrodes. Additionally, the HEAM helped elucidate the deficiencies of flow distribution in interdigitated flow fields (IFFs) and suggested designing wider manifolds and/or shorter channels improve the flow distribution. Moreover, the HEAM suggested shallower and/or wider channels, and more permeable electrodes enhance the flow penetration rate above the channels. Finally, the HEAM showed that the average penetration depth in the electrode above the ribs (hpen) was the critical parameter in the fluid‐flow modeling of IFFs and was inversely proportional to the permeability. Hence, there is a trade‐off between the pump power loss and hpen when configuring electrode permeability.
The chlorine evolution reaction (CER) and oxygen evolution reaction (OER) occur simultaneously due to the low difference (0.13 V) in their standard potentials. RuO2 is the state‐of‐art ...electrocatalyst used for both OER and CER. The activity and selectivity of different RuO2 low‐index facets, namely (100), (110), (111), (001) and (101), are investigated through ab‐initio density functional theory (DFT) based calculations. The selectivity of different facets is explored in a mixed OER‐CER region by combining Pourbaix diagrams and linear scaling relationships. The difference in limiting overpotential of OER and CER is identified as the selectivity descriptor (SDCER). The most CER‐ and OER‐selective facets are found to be (101) (SDCER=0.39 V) and (001) (SDCER=0.14 V), respectively. The understanding of facet dependent CER selectivity in RuO2 can be extended as a design strategy to modulate OER and CER activity and selectivity as per design requirements.
Controlling activity and selectivity: The free energy difference between −OCl and −OOH intermediate acts as the chlorine evolution reaction (CER) selectivity descriptor in RuO2. The most selective facet for CER and oxygen evolution reaction (OER) are (101) and (001) in RuO2, respectively.
Functional changes in spatial genome organization during human development are poorly understood. Here we report a comprehensive profile of nuclear dynamics during human cardiogenesis from ...pluripotent stem cells by integrating Hi-C, RNA-seq and ATAC-seq. While chromatin accessibility and gene expression show complex on/off dynamics, large-scale genome architecture changes are mostly unidirectional. Many large cardiac genes transition from a repressive to an active compartment during differentiation, coincident with upregulation. We identify a network of such gene loci that increase their association inter-chromosomally, and are targets of the muscle-specific splicing factor RBM20. Genome editing studies show that TTN pre-mRNA, the main RBM20-regulated transcript in the heart, nucleates RBM20 foci that drive spatial proximity between the TTN locus and other inter-chromosomal RBM20 targets such as CACNA1C and CAMK2D. This mechanism promotes RBM20-dependent alternative splicing of the resulting transcripts, indicating the existence of a cardiac-specific trans-interacting chromatin domain (TID) functioning as a splicing factory.
Nonstoichiometric CeO2 and Ce0.25Zr0.75O2 nanoparticles with varying surface concentrations of Ce3+ were synthesized. Their surface Ce3+ concentration was measured by XPS, and their surface oxygen ...vacancy concentrations and grain size were estimated using Raman spectroscopy. The surface oxygen vacancy concentration was found to correlate well with grain size and surface Ce3+ concentration. When incorporated into a Nafion polymer electrolyte membrane (PEM), the added nonstoichiometric ceria nanoparticles effectively scavenged PEM-degradation-inducing free radical reactive oxygen species (ROS) formed during fuel cell operation. A 3-fold increase in the surface oxygen vacancy concentration resulted in an order of magnitude enhancement in the efficacy of free radical ROS scavenging by the nanoparticles. Overall, the macroscopic PEM degradation mitigation rate was lowered by up to 2 orders of magnitude using nonstoichiometric ceria nanoparticles with high surface oxygen vacancy concentrations
PDF
