Stability and durability of PtRu catalysts supported on carbon nanofibers for direct methanol fuel cells

The chemical stability and durability of PtRu catalysts supported on carbon nanofibers (CNFs) for the anode electrode of a direct methanol fuel cell (DMFC) are investigated by Pt and Ru dissolution tests in sulfuric acid and long-term performance tests of a single cell discharging at a constant current density of 150 mA cm⁻² for approximately 2000 h. A CNF with a herringbone-type structure, which is characterized by the alignment of graphene symmetrically angled to the fiber axis, was selected as the catalyst support because it has an edge-rich surface and a high surface area. In the metal dissolution test, the PtRu/CNF catalysts showed 1.5–2 times lower Ru leaching than a tested commercial catalyst (supported on activated carbon). The results of long-term performance tests also prove the higher durability of the anode catalyst compared with the commercial catalyst, when the anode polarization is compared before and after operation for 2000 h. Some analytical measurements, including X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy were conducted to study the degradation of the catalyst activity.     

Effects of acid,alkaline, and seawater aging on the mechanical and thermomechanical properties of glass fiber/epoxy composites filled with carbon nanofibers

Owing to the superior corrosion resistance, fiber-reinforced polymer (FRP) composites are the prime choice of structural materials for various marine and chemical industries, where there is a long-term direct contact of the components takes place with corrosive fluids. In this present work, glass fiber/epoxy (GE) composites have been fabricated with and without carbon nanofibers (CNFs), and aging has been carried out in acidic (pH = 1), seawater (pH = 8.2), and alkaline (pH = 13) solutions for 150 days. The resistance of CNF-filled GE composites toward the corrosive fluids has been evaluated in terms of alteration in the mechanical (flexural), microstructural (fractography analysis by field emission scanning electron microscope), and thermomechanical (dynamic mechanical analysis) behavior of the materials. It is revealed that as the immersion time increases, there is a continuous decrement in flexural strength and modulus, and glass-transition temperature (T_g) of all the materials in all these solutions. Compared to the 1% CNF-filled GE composite, control GE composite showed more degradation in the case of alkaline aging and seawater aging. Maximum reduction (56%) in the strength of GE composite was observed due to 150 days of alkaline aging. However, the control GE composite showed better resistance to the acidic solution than that of CNF-filled GE composite. Possible failure modes, changes in the chemistry of the material due to aging have been studied by fractography analysis. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48434.     

Probability logic and optimization SAT: The PSAT and CPA models

Both probabilistic satisfiability (PSAT) and the check of coherence of probability assessment (CPA) can be considered as probabilistic counterparts of the classical propositional satisfiability problem (SAT). Actually, CPA turns out to be a particular case of PSAT; in this paper, we compare the computational complexity of these two problems for some classes of instances. First, we point out the relations between these probabilistic problems and two well known optimization counterparts of SAT, namely Max SAT and Min SAT. We then prove that Max SAT with unrestricted weights is NP-hard for the class of graph formulas, where Min SAT can be solved in polynomial time. In light of the aforementioned relations, we conclude that PSAT is NP-complete for ideal formulas, where CPA can be solved in linear time.     

Translation among CNFs, characteristic models and ordered binary decision diagrams

We consider translation among conjunctive normal forms (CNFs), characteristic models, and ordered binary decision diagrams (OBDDs) of Boolean functions. It is shown in this paper that Horn OBDDs can be translated into their Horn CNFs in polynomial time. As for the opposite direction, the problem can be solved in polynomial time if the ordering of variables in the resulting OBDD is specified as an input. In case that such ordering is not specified and the resulting OBDD must be of minimum size, its decision version becomes NP-complete. Similar results are also obtained for the translation in both directions between characteristic models and OBDDs. We emphasize here that the above results hold on any class of functions having a basis of polynomial size.     

纳米炭纤维的表面润湿行为

采用动态渗透法研究了不同结构的纳米炭纤维(Carbonnanofibers,CNF)表面润湿性以及在不同溶剂中润湿性的变化和表面改性对其润湿性的影响。结果表明,生长条件如催化剂组成、碳源等对CNF的表面性质有显著影响,并最终决定其在溶剂中的润湿能力。以Fe/γ-Al2O3为催化剂,以C2H4为碳源得到的CNF在水中的润湿性能最差;而以Ni/γ-Al2O3为催化剂,CO为碳源得到的CNF在环己烷中的润湿性能最好。CNF在不同溶剂中的相对接触角测定表明CNF是一种表面非极性较强的材料。设CNF在润湿性能最好的环己烷中的接触角为0°,则CNF在水,丙酮,乙醇中的相对接触角分别为81.6°,45.2°,24.8°。不同的表面改性手段可对CNF的表面性质进行调变以控制其在不同溶剂中的润湿行为。在浓硝酸中液相氧化可提高其对水溶液和环己烷的润湿性能;在氩气中的高温热处理可提高其对水溶液的润湿性能,但降低了对环己烷的润湿能力;而在过氧化氢溶液中的处理则同时降低了对水溶液以及环己烷的润湿能力。     

Conformational Insights into the Control of CNF1 Toxin Activity by Peptidyl-Prolyl Isomerization: A Molecular Dynamics Perspective

The cytotoxic necrotizing factor 1 (CNF1) toxin from uropathogenic Escherichia coli constitutively activates Rho GTPases by catalyzing the deamidation of a critical glutamine residue located in the switch II (SWII). In crystallographic structures of the CNF1 catalytic domain (CNF1^CD), surface-exposed P768 and P968 peptidyl-prolyl imide bonds (X-Pro) adopt an unusual cis conformation. Here, we show that mutation of each proline residue into glycine abrogates CNF1^CD in vitro deamidase activity, while mutant forms of CNF1 remain functional on RhoA in cells. Using molecular dynamics simulations coupled to protein-peptide docking, we highlight the long-distance impact of peptidyl-prolyl cis-trans isomerization on the network of interactions between the loops bordering the entrance of the catalytic cleft. The energetically favorable isomerization of P768 compared with P968, induces an enlargement of loop L1 that fosters the invasion of CNF1^CD catalytic cleft by a peptide encompassing SWII of RhoA. The connection of the P968 cis isomer to the catalytic cysteine C866 via a ladder of stacking interactions is alleviated along the cis-trans isomerization. Finally, the cis-trans conversion of P768 favors a switch of the thiol side chain of C866 from a resting to an active orientation. The long-distance impact of peptidyl-prolyl cis-trans isomerizations is expected to have implications for target modification.     

Electrospun zeolitic imidazolate framework‐derived nitrogen‐doped carbon nanofibers with high performance for lithium‐sulfur batteries

Three‐dimensional (3D) nitrogen‐doped carbon nanofibers (N‐CNFs) which were originating from nitrogen‐containing zeolitic imidazolate framework‐8 (ZIF‐8) were obtained by a combined electrospinning/carbonization technique. The pores uniformly distributed in N‐CNFs result in the improvement of electrical conductivity, increasing of BET surface area (142.82 m² g^?1), and high porosity. The as‐synthesized 3D free‐standing N‐CNFs membrane was applied as the current collector and binder free containing Li₂S₆ catholyte for lithium‐sulfur batteries. As a novel composite cathode, the free‐standing N‐CNFs/Li₂S₆ membrane shows more stable electrochemical behavior than the CNFs/Li₂S₆ membrane, exhibiting a high first‐cycle discharge specific capacity of 1175 mAh g^?1at 0.1 C and keeping discharge specific capacity of 702 mAh g^?1 at higher rate. More importantly, as the sulfur mass in cathodes was increased at 7.11 mg, the N‐CNFs/Li₂S₆ membrane delivered 467 mAh g^?1after 150 cycles at 0.2 C. The excellent electrochemical properties of N‐CNFs/Li₂S₆ membrane can be ascribed to synergistic effects of high porosity and nitrogen‐doping in N‐CNFs from carbonized ZIF‐8, illustrating collective effects of physisorption and chemisorption for lithium polysulfides in discharge‐charge processes.     

k-LSAT (k≥3)是NP-完全的

合取范式(conjunctive normal form,简称CNF)公式F是线性公式,如果F中任意两个不同子句至多有一个公共变元.如果F中的任意两个不同子句恰好含有一个公共变元,则称F是严格线性的.所有的严格线性公式均是可满足的,而对于线性公式类LCNF,对应的判定问题LSAT仍然是NP-完全的.LCNF_≥k是子句长度大于或等于k的CNF公式子类,判定问题LSA(≥k)的NP-完全性与LCNF(≥k)中是否含有不可满足公式密切相关.即LSAT_≥k的NP-完全性取决于LCNF_≥k是否含有不可满足公式.S.Porschen等人用超图和拉丁方的方法构造了LCNF_≥3和LCNF_≥4中的不可满足公式,并提出公开问题:对于k≥5,LCNF_≥k是否含有不可满足公式?将极小不可满足公式应用于公式的归约,引入了一个简单的一般构造方法.证明了对于k≥3,k-LCNF含有不可满足公式,从而证明了一个更强的结果:对于k≥3,k-LSAT是NP-完全的.     

并行计算：提高SAT问题求解效率的有效方法

基于拟物拟人思想的Solar算法是一个求解SAT问题的快速算法.实验和理论分析表明,Solar算法具有易并行化的特性.将Solar算法并行化可大幅度地提高求解SAT问题的效率.