Anion exchange membranes are an important component in alkaline electrochemical energy conversion and storage devices, and their alkaline stability plays a crucial role for the long-term use of these ...devices. Herein, a systematic study was conducted for the analysis of polymer backbone chemical stability in alkaline media. Nine representative polymer structures including poly(arylene ether)s, poly(biphenyl alkylene)s, and polystyrene block copolymers were investigated for their alkaline stability. Polymers with aryl ether bonds in their repeating unit showed poor chemical stability when treated with KOH and NaOCH3 solutions, whereas polymers without aryl ether bonds e.g., poly(biphenyl alkylene)s and polystyrene block copolymers remained stable. Additional NMR studies and density functional theory (DFT) calculations of small molecule model compounds that mimic the chemical structures of poly(arylene ether)s confirmed that electron-withdrawing groups near to the aryl ether bonds in the repeating unit accelerate chemical degradation. Results from this study suggest that the use of all-carbon-based polymer repeating units (i.e., polymers not bearing aryl ether bonds) can enhance long-term alkaline stability of anion exchange membranes in electrochemical energy devices.
Acid retention of phosphoric acid (PA)-doped proton exchange membranes (PEMs) is one of the critical factors that determine the durability of high temperature PEM fuel cells. However, the mechanism ...of PA loss in the PEMs in the presence of water is obscure in the context of the energetics of the PA cluster. Here, we study the energetics of PA–benzimidazole acid–base and biphosphate–ammonium ion pairs using density functional theory calculations and 31 P NMR experiments to propose a novel PA loss mechanism. The results suggest that the removal of the PA from the membrane does not occur due to the strong interaction of PA–water, but due to the incapability of the base polymers to hold the water and PA beyond a certain level. Significantly higher interaction in the biphosphate–ammonium ion pair shifts the equilibrium PA composition in the PA cluster to higher values, which minimizes the PA loss in the presence of water. Introducing high interaction between base, water, and PA molecules provides a path for better high temperature PEM design with excellent acid retention capabilities.
The effect of the Pt shell thickness on the oxygen reduction reaction (ORR) of a Pd@Pt core‐shell catalyst was studied using surface science technics and computational approaches. We found Pt shells ...on Pd rods to be negatively charged because of charge transfer from the Pd substrate when the shell thicknesses were 0.5 or 1 monolayer (ML). The activities of the ORR of the model surface with a Pt shell of 0.5 or 1 ML were similar and more than twice the activities of a Pt/C or Pt rod. The relationship between the ORR activity and the thickness of the Pt shell was the exact opposite of the relationship between the Pt binding energy and the Pt shell thickness. The indication was that more negatively charged Pt had higher ORR activity. Density functional theory calculations confirmed that a single layer of Pt atoms located on Pd was negatively charged compared to pure Pt and resulted in a lower barrier to the rate‐limiting step of the ORR.
Pt@Pd core‐shell catalysts containing a single shell layer show a better catalytic performance because the top Pt layer atoms located over the Pd atoms are more negatively charged than pure Pt, which lowers the barrier to the rate‐limiting step of the oxygen reduction reaction.
Density functional theory (DFT) calculations were conducted to investigate mechanistic details of ethanol‐to‐butadiene conversion reaction over MgO or ZnO catalyst. We evaluated the Lewis acidity and ...basicity of MgO and ZnO and found that ZnO had the stronger Lewis acidity and basicity than MgO. Potential energy surfaces of ethanol‐to‐butadiene conversion, which included relevant transition states and intermediates, were computed in detail following the generally accepted mechanism reported in the literature, where such mechanism included ethanol dehydrogenation, aldol condensation, Meerwein‐Pondorf‐Verley reduction, and crotyl alcohol dehydration. DFT results showed that ethanol dehydrogenation was the rate‐limiting step of overall reaction when the reaction was catalyzed by MgO. Also, DFT results showed that ethanol dehydrogenation occurred more easily on ZnO than on MgO, where such a result correlated with the stronger Lewis acidity of ZnO. In addition, we computed ethanol dehydration, which generates ethylene, one of the major undesired side reaction products for butadiene formation. DFT results showed that ZnO favored dehydrogenation over dehydration, while MgO favored dehydration.
Density functional theory (DFT) calculations were conducted to investigate mechanistic details of ethanol‐to‐butadiene conversion reaction over MgO or ZnO catalyst. DFT results showed that ethanol dehydrogenation occurred more easily on ZnO than on MgO, where such a result correlated with the stronger Lewis acidity of ZnO. In addition, we computed ethanol dehydration, which generates ethylene, one of the major undesired side reaction products for butadiene formation. DFT results showed that ZnO favored dehydrogenation over dehydration, while MgO favored dehydration.
The first trivalent and pentavalent tricarbabismatranes were synthesized by the reaction of N(CH
2
{2-LiC
6
H
4
})
3
with BiCl
3
and subsequent reaction with XeF
2
, respectively. The trivalent ...bismatrane was easily oxidized by air, while the pentavalent bismatrane difluoride was relatively stable to air. A similar pentavalent bismatrance dichloride was prone to C-Cl bond reductive elimination even at room temperature.
The first trivalent and pentavalent tricarbabismatranes bearing the N(CH
2
C
6
H
4
−
)
3
ligand were synthesized.
We have successfully isolated and characterized the zinc carbamate complex (phen)Zn(OAc)(OC(=O)NHPh) (1; phen=1,10‐phenanthroline), formed as an intermediate during the Zn(OAc)2/phen‐catalyzed ...synthesis of organic carbamates from CO2, amines, and the reusable reactant Si(OMe)4. Density functional theory calculations revealed that the direct reaction of 1 with Si(OMe)4 proceeds via a five‐coordinate silicon intermediate, forming organic carbamates. Based on these results, the catalytic system was improved by using Si(OMe)4 as the reaction solvent and additives like KOMe and KF, which promote the formation of the five‐coordinated silicon species. This sustainable and effective method can be used to synthesize various N‐aryl and N‐alkyl carbamates, including industrially important polyurethane raw materials, starting from CO2 under atmospheric pressure.
We have isolated and analyzed the reactivity of the key intermediate 1 formed during the Zn(OAc)2/phen‐catalyzed synthesis of organic carbamates starting from amines, CO2, and Si(OMe)4. The reaction mechanism was analyzed by using DFT. The catalytic system was modified and various N‐aryl and N‐alkyl carbamates that could be used as industrially important polyurethane raw materials were synthesized by using atmospheric CO2.
Alkaline stability of benzyl trimethylammonium (BTMA)-functionalized polyaromatic membranes was investigated by computational modeling and experimental methods. The barrier height of hydroxide ...initiated aryl-ether cleavage in the polymer backbone was computed to be 85.8 kJ/mol, a value lower than the nucleophilic substitution of the α-carbons on the benzylic position of BTMA cationic functional group, computed to be 90.8 kJ/mol. The barrier heights of aryl–aryl cleavage (polymer backbone) are 223.8–246.0 kJ/mol. The computational modeling study suggests that the facile aryl–ether cleavage is not only due to the electron deficiency of the aryl group but also due to the low bond dissociation energy arising from the ether substituent. Ex situ degradation studies using Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) spectroscopy indicated that 61% of the aryl–ether groups degraded after 2 h of treatment in 0.5 M NaOH at 80 °C. BTMA cationic groups degraded slowly over 48 h under the same conditions. In situ degradation studies validate the calculated results: anion exchange membrane fuel cells and water electrolyzer using poly(arylene ether) membranes exhibit a catastrophic, premature failure during lifetime tests, while no sudden performance loss is observed with an ether-free poly(phenylene) membrane. Despite the gradual performance loss due to the degradation of BTMA cation functional group, the membrane electrode assembly using the poly(phenylene) membrane exhibited a lifetime of >2000 h in the alkaline water electrolyzer mode at 50 °C.
Batteries based on zinc (Zn) chemistry offer a great opportunity for large-scale applications owing to their safety, cost-effectiveness, and environmental friendliness. However, the poor Zn ...reversibility and inhomogeneous electrodeposition have greatly impeded their practical implementation, stemming from water-related passivation/corrosion. Here, we present a multifunctional electrolyte comprising gamma-butyrolactone (GBL) and Zn(BF4)2·xH2O to resolve these intrinsic challenges. The systematic results confirm that water reactivity toward a Zn anode is minimized by forcing GBL solvents into the Zn2+ solvation shell and constructing a fluorinated interphase on the Zn anode surface via anion decomposition. Furthermore, NMR was selected as an auxiliary testing protocol to elevate and understand the role of electrolyte composition in building the interphase. The combined factors in synergy guarantee high Zn reversibility (average Coulombic efficiency is 99.74%), high areal capacity (55 mAh/cm2), and high Zn utilization (∼91%). Ultimately, these merits enable the Zn battery utilizing a VO2 cathode to operate smoothly over 5000 cycles with a low-capacity decay rate of ∼0.0083% per cycle and a 0.23 Ah VO2/Zn pouch cell to operate over 400 cycles with a capacity retention of 77.3%.
To understand the mechanistic details of the catalytic conversion of ethanol to 1,3-butadiene on metal oxides, both the main reaction and the side reactions should be clarified. Seven side reactions ...on an MgO catalyst were examined using density functional theory calculations. They were: the condensation of ethanol involving dehydration, which generates diethyl ether; condensation between ethanol and acetaldehyde, which generates ethyl acetal; reduction of acetaldol, which generates 1,3-butanediol (1,3-BDO); dehydration of 1,3-BDO, which generates methyl ethyl ketone; hydrogenation of crotonaldehyde, which generates
n
-butanol; isomerization of crotyl alcohol, which generates butanal; and dehydrogenation and decarboxylation of acetaldol, which generate acetone. Because the ethanol-to-butadiene conversion proceeds via several reaction steps, which are catalyzed on Lewis acidic and/or basic sites, increasing the efficiency of a reaction step in the main reaction path would also increase side reaction paths of other reaction steps.
Investigation of two common explosives such as cyclonite (RDX) and cyclotetramethylenetetranitramine (HMX) using a mass spectrometer with ultrahigh resolution and accuracy has not been ...comprehensively performed. Here, ultrahigh mass accuracy 15‐T Fourier transform–ion cyclotron resonance mass spectrometry (FT‐ICR MS) spectra were utilized to comprehensively characterize the adduct ions of RDX and HMX. Two different ionization sources such as a conventional electrospray ionization (ESI) source and a chip‐based static nano‐ESI source were used to investigate the adduct ions of RDX and HMX. The ESI‐MS analyses of two explosives in negative ion mode provide some adduct ions of RDX and HMX even without prior addition of their corresponding anions. A total of six types of adduct ion were characterized: M + Cl−, M + HCOO−, M + NO2−, M + CH3COO−, M + NO3−, and M + C3H5O3−, where M is either RDX or HMX. The ultrahigh accuracy of the 15‐T FT‐ICR MS was utilized to distinguish two closely spaced peaks representing the monoisotopic M + NO2− and second isotopic M + HCOO− ions, thereby enabling the discovery of a M + NO2− adduct ion in the ESI analysis of RDX or HMX. M + NO2− and M + CH3COO− adduct ions were only observed when using a static nano‐ESI source. It is the first report explaining the discovery of M + NO2− adduct ion in the ESI‐MS analyses of RDX and HMX.
