Phosphoric acid (PA)-doped high temperature polymer electrolyte membranes (HT-PEMs) are crucial materials for HT-PEM fuel cells (HT-PEMFCs). However, the development of HT-PEMs suffers from the ...trade-off between proton conductivity and mechanical strength. High proton conductivity requires a high doping level of PA, and PA acts as a plasticizer that reduces the mechanical properties. Here, a new strategy is employed to address the unresolved challenges; the strategy is to graft poly(1-vinylimidazole) as PA doping sites on the polysulfone backbone. This is achieved via atom transfer radical polymerization. High proton conductivity is achieved because of the formation of micro-phase separated structures, and the mechanical properties are retained because of the reduced plasticizing effect, which is caused by the separation of PA adsorption sites and the polymer backbone. The prepared PA-doped membranes have excellent proton conductivity of 127 mS cm−1 at 160 °C and outstanding tensile strength of 7.94 MPa. Meanwhile, single H2-O2 cell performance with the optimized membrane is impressive, reaching a peak power density of 559 mW cm−2 at 160 °C. More importantly, this work provides new insight into solving the trade-off between proton transport and mechanical strength for PA-doped HT-PEMs.
•Poly(1-vinylimidazole) grafted on polysulfone membranes were synthesized.•Phosphoric acid doped membranes possess micro-phase separation structure.•The membranes display enhanced conductivity with increasing length of side chains.•The tensile strength of phosphoric acid doped membranes still remains 7.94 MPa.•The single cell with membranes shows a peak power density of 559 mW cm−2 at 160 °C.
The development of high temperature proton exchange membranes (HT-PEMs) with high proton conduction and excellent mechanical properties remains a challenge. Herein, the graphitic carbon nitride (CN) ...nanosheets were successfully introduced into poly(ether sulfones)-poly(vinyl pyrrolidone) polymer matrix to prepare composite membrane through a facile blending method. The synthesized CN nanosheets were characterized by scanning electron microscope (SEM), Transmission electron microscopy (TEM), Raman spectrum, and X-ray photoelectron spectroscopy (XPS). The proton conductivity of the composite membrane was improved up to 36% (0.104 S cm−1) after incorporating CN nanosheets at 160 °C, due to increased PA doping level and faster proton dissociation. Meanwhile, the tensile strength of the composite membrane is increased of 60% (6.0 MPa) compared to that of pristine membrane, because of the physical reinforced effect from the 2D structure of CN. Furthermore, the single cell fabricated with the optimized membrane exhibits a peak power density of 512 mW cm−2 at the temperature of 160 °C for 200 h with no obvious loss of current density.
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•Graphitic carbon nitride (CN) nanosheets were incorporated into PES-PVP membranes.•The composite membranes displayed enhanced mechanical properties.•The proton conductivity was improved with an optimized content of CN.•The HT-PEMFC with 0.5 wt% CN shows a peak power density of 512 mW cm−2 at 160 °C.
Since 2016 October, the active galaxy PKS 2247-131 has undergone a γ-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a ...relativistic jet in PKS 2247-131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ≃34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247-131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247-131.
Although comprehensive progress has been made in the area of coordination polymer (CP)/metal–organic framework (MOF)-based proton-conducting materials over the past decade, searching for a CP/MOF ...with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton-conducting CP, (NH4)3Zr(H2/3PO4)3 (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered charge-balancing NH4 + cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid–base pairs (N–H···O–P), leading to a stable anhydrous proton conductivity of 1.45 × 10–3 S·cm–1 at 180 °C, one of the highest values among reported intermediate-temperature proton-conducting materials. First-principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH4 + and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H2/O2 fuel cell, which showed a record-high electrical power density of 12 mW·cm–2 at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power loss.
The microstructures of catalyst layers (CLs) provide the paths for phosphoric acid (PA) invasion and decide the amount and distribution of PA in CLs, which is essential to improve the performance and ...stability of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). In this work, the CLs with different microstructures are constructed and the effects of Pt loading on the performance and degradation of HT-PEMFCs are studied. The results show the CLs with flat surface slow down the process of PA invasion and well-developed pore structures promote the redistribution of PA, which results in low mass transfer resistance. Therefore, the peak power density of HT-PEMFC based on CLs prepared by ultrasonic-spraying is 1.4 times than that by screen-printing, while the performance degradation is only 11% after accelerated stress test of 30,000 cycles with the Pt loading of 0.5 mg cm−2. Distribution of relaxation times analysis is used to assist the electrochemical impedance spectroscopy to further distinguish the different physicochemical process within cells. The result reveals that mass transfer is affected greatly by the effects of microstructures and Pt loadings, and gets deterioration gradually with the invasion of PA into CLs, which not only makes Pt particle growth but decreases kinetics of oxygen reduction reaction.
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•Understanding the impact of electrode microstructures on Pt utilization.•Distribution of relaxation times analyses were performed to identify loss processes.•Analysis of loss processes dependencies on electrode microstructures, Pt loading.•The deterioration of mass transfer leads to performance degradation.
We present extensive optical observations of a Type IIn supernova (SN IIn) 2010j1 for the first 1.5 years after its discovery. The UBVRI light curves demonstrated an interesting two-stage evolution ...during the nebular phase, which almost flatten out after about 90 days from the optical maximum. These lines can be well decomposed into a narrow component and an intermediate-width component. The intermediate-width component showed a steady increase in both strength and blueshift with time until t ~ 400 days after maximum, but it became less blueshifted at t ~ 500 days, when the line profile appeared relatively symmetric again. Considering a slow-moving stellar wind (e.g., ~28 km s-l) inferred for the preexisting, dense CSM shell and the extremely high mass-loss rate (1-2 M sub(odot) yr super(-1)), we suggest that the progenitor of SN 2010jl might have experienced a red supergiant stage and may explode finally as a post-red supergiant star with an initial mass above 30-40 M sub(odot).
Abstract
We presented a detailed analysis of progenitor properties of type IIP supernova 2017eaw in NGC 6946, based on the pre-explosion images and early-time observations obtained immediately after ...the explosion. An unusually red star, with MF814W = −6.9 mag and mF606W − mF814W = 2.9 ± 0.2 mag, can be identified at the SN position in the pre-discovery Hubble Space Telescope (HST) images taken in 2016. The observed spectral energy distribution of this star, covering the wavelength of 0.6–2.0 $\ \mathrm{\mu m}$, matches that of an M4-type red supergiant (RSG) with a temperature of about 3550 K. These results suggest that SN 2017eaw has a RSG progenitor with an initial mass of 12 ± 2 M⊙. The absolute F814W-band magnitude of this progenitor star is found to evolve from −7.2 mag in 2004 to −6.9 mag in 2016. Such a dimming effect is, however, unpredicted for an RSG in its neon/oxygen burning phase when its luminosity should modestly increase. The spectrum of SN 2017eaw taken a few hours after discovery clearly shows a narrow Hα emission feature blueshifted by ∼160 km s−1. This narrow component disappeared in the spectrum taken two days later, suggesting the presence of a circumstellar material (CSM) shell (i.e. at a distance of <2.1–4.3 × 1014 cm). Combining the inferred distance with the expansion velocity of the CSM, we suggest that the progenitor of SN 2017eaw should have experienced violent mass-loss at about 1–2 yr prior to explosion, perhaps invoked by pulsational envelop ejection. This mechanism may help explain its luminosity decline in 2016 as well as the lack of detections of RSGs with relatively higher initial mass as progenitors of SNe IIP.
ABSTRACT We present well-sampled optical observations of the bright Type Ia supernova (SN Ia) SN 2011fe in M101. Our data, starting from ∼16 days before maximum light and extending to ∼463 days after ...maximum, provide an unprecedented time series of spectra and photometry for a normal SN Ia. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a tn law, with the index n being close to 2.0 in the VRI bands but slightly larger in the U and B bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNe Ia with similar Δm15(B), suggestive of less trapping of high-energy photons in the ejecta. Moreover, the U-band light curve shows a notably faster decline at late phases (t 100-300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O i λ7773 and the Ca ii NIR triplet, but only barely detectable in Si ii 6355. This difference can be caused by either an ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.
Stars with initial masses in the range of 8–25 solar masses are thought to end their lives as hydrogen-rich supernovae (SNe II). Based on the pre-explosion images of Hubble space telescope (HST) and ...Spitzer space telescope, we place tight constraints on the progenitor candidate of type IIP SN 2023ixf in Messier 101. Fitting of the spectral energy distribution (SED) of its progenitor with dusty stellar spectral models results in an estimation of the effective temperature as 3091
−258
+422
K. The luminosity is estimated as lg(
L
/
L
⊙
)∼4.83, consistent with a red supergiant (RSG) star with an initial mass of 12
−1
+2
M
⊙
. The derived mass loss rate (6×10
−6
−9×10
−6
M
⊙
yr
−1
) is much lower than that inferred from the flash spectroscopy of the SN, suggesting that the progenitor experienced a sudden increase in mass loss when approaching the final explosion. In the infrared bands, significant deviation from the range of regular RSGs in the color-magnitude diagram and period-luminosity space of the progenitor star indicates enhanced mass loss and dust formation. Combined with new evidence of polarization at the early phases of SN 2023ixf, such a violent mass loss is likely a result of binary interaction.
Abstract
The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This ...very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at −13.6 days relative to the
B
-band maximum light, or ∼5 days after the estimated explosion—the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si
ii
λ
6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.
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