Ternary Heterostructural Pt/CNx/Ni as a Supercatalyst for Oxygen Reduction

Recent Approaches for Cleaving the C─C Bond During Ethanol Electro-Oxidation Reaction

Direct ethanol fuel cells (DEFCs) play an indispensable role in the cyclic utilization of carbon resources due to its high volumetric energy density, high efficiency, and environmental benign character. However, owing to the chemically stable carbon-carbon (C─C) bond of ethanol, its incomplete electrooxidation at the anode severely inhibits the energy and power density output of DEFCs. The efficiency of C─C bond cleaving on the state-of-the-art Pt or Pd catalysts is reported as low as 7.5%. Recently, tremendous efforts are devoted to this field, and some effective strategies are put forward to facilitate the cleavage of the C─C bond. It is the right time to summarize the major breakthroughs in ethanol electrooxidation reaction. In this review, some optimization strategies including constructing core-shell nanostructure with alloying effect, doping other metal atoms in Pt and Pd catalysts, engineering composite catalyst with interface synergism, introducing cascade catalytic sites, and so on, are systematically summarized. In addition, the catalytic mechanism as well as the correlations between the catalyst structure and catalytic efficiency are further discussed. Finally, the prevailing limitations and feasible improvement directions for ethanol electrooxidation are proposed.

Accelerating Ethanol Complete Electrooxidation via Introducing Ethylene as the Precursor for the C-C Bond Splitting

The crucial issue restricting the application of direct ethanol fuel cells (DEFCs) is the incomplete and sluggish electrooxidation of ethanol due to the chemically stable C-C bond thereof. Herein, a unique ethylene-mediated pathway with a 100 % C1-selectivity for ethanol oxidation reaction (EOR) is proposed for the first time based on a well-structured Pt/Al2 O3 @TiAl catalyst with cascade active sites. The electrochemical in situ Fourier transform infrared spectroscopy (FTIR) and differential electrochemical mass spectrometry (DEMS) analysis disclose that ethanol is primarily dehydrated on the surface of Al2 O3 @TiAl and the derived ethylene is further oxidized completely on nanostructured Pt. X-ray absorption and density functional theory (DFT) studies disclose the Al component doped in Pt nanocrystals can promote the EOR kinetics by lowering the reaction energy barriers and eliminating the poisonous species. Strikingly, Pt/Al2 O3 @TiAl exhibits a specific activity of 3.83 mA cm-2 Pt , 7.4 times higher than that of commercial Pt/C and superior long-term durability.

Engineering carbon semi-tubes supported platinum catalyst for efficient oxygen reduction electrocatalysis

Innovation of catalyst structure is extremely important to develop the high-performance electrocatalysts for oxygen-reduction reaction (ORR). Herein, nitrogen-doped carbon semi-tube (N-CST) is used as a functional support for stabilizing the microwave-reduced Pt nanoparticles with an average size of ∼2.8 nm to synthesize the semi-tubular Pt/N-CST catalyst. The contribution of interfacial Pt-N bond between N-CST support and Pt nanoparticles with electrons transfer from N-CST support to Pt nanoparticles is found by electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) spectroscopy. This bridged Pt-N coordination can simultaneously help ORR electrocatalysis and promote electrochemical stability. As a result, the innovative Pt/N-CST catalyst exhibits excellent catalytic performance, realizing ORR activity and electrochemical stability superior to the commercial Pt/C catalyst. Furthermore, density functional theoretical (DFT) calculations suggest that the interfacial Pt-N-C site with unique affinity of O∗ + OH∗ can provide new active routes for the enhanced electrocatalytic ORR capacity.

Study on the Alternative Solvent of Methylbenzene in the Total Acid Number Titration of Current Jet Fuels

Evaluation of the acidic characteristics of a jet fuel, especially for the total acid number (TAN), is of great significance to ensure flight safety. Methylbenzene is commonly used as the titration solvent; however, it is poisonous and harmful to the environment. It is highly desirable to develop an alternative solvent for methylbenzene to extract the acidic compounds from the jet fuel during the determination of the TAN. Here, we develop a desirable alternative solvent of a mixed ethanol-water solution with the volume ratio of ethanol to water of 99:1, which exhibits a value of TAN similar to that of the solvent of methylbenzene in potentiometric titration and acid-base titration methods. The TAN value derived from the different titration solvents was in the order of 2.96 μg KOH g^-1 (V _cyclohexane/V _isopropanol/V _water = 100:99:1) > 2.68 μg KOH g^-1 (V _{methylbenzene}/V _isopropanol/V _water = 100:99:1) ≈ 2.6 μg KOH g^-1 (V _{absolute ethanol}/V _water = 99:1) > 2.34 μg KOH g^-1 (V _isopropanol/V _water = 99:1). The current report presents a nontoxic and eco-friendly alternative solvent for methylbenzene, which may open up an avenue for evaluating the TAN of jet fuels.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO₂ /N₂ reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Binary System of Polyethylene Glycol 200 (1) + 3-Dimethylamino-1-propylamine (2) for CO2 Absorption: Thermophysical Properties and Spectroscopic Study

As the concentration of CO₂ in the atmosphere keeps increasing, the development of a highly efficient CO₂ absorbent is highly desired. In this work, a binary mixture system of polyethylene glycol 200 (PEG 200) (1) + 3-dimethylamino-propylamine (DMAPA) (2) was used for CO₂ absorption. Considering the importance of thermophysical properties to binary solutions, the densities and viscosities of the PEG 200 (1) + DMAPA (2) mixture were measured at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and atmospheric pressure over the entire composition range. Based on the density and viscosity data, the excess properties and viscous flow thermodynamic parameters were calculated, respectively. To obtain the coefficients and to estimate the standard deviations between the experimental and calculated quantities, the excess molar volume (V _m ^E), the viscosity deviation (Δη), and the excess Gibbs free energies of activation for viscous flow (ΔG* ^E) were fitted to the Redlich-Kister equation. Furthermore, based on the results of UV-vis, FTIR, and ¹H NMR spectra, the intermolecular interaction of PEG 200 and DMAPA was discussed. Particularly, a strong intermolecular bonding is formed when the molar ratio of PEG 200 to DMAPA is about 1:2 because of the excess molar volume (V _m ^E). On that account, a mixture of PEG 200 and DMAPA in a ratio of 1:2 was used for studying CO₂ absorption, and a CO₂ absorption of about 0.19 g per gram of absorbent was achieved at room temperature and atmosphere.

Influence of Foam Characteristics on the Aviation Coolants' Pollution Degree

The particulate contamination degree of aviation coolants (ACs) is overestimated commonly because the bubbles in ACs are erroneously recognized as particulate contaminants during the measurement process. In this work, the factors that influence the foam behavior and contamination degree of ACs are investigated. It is evidenced that the foam behavior of ACs is basically unaffected by the ratio of glycol to water of the base solution, which, however, is highly influenced by the organic additive. Also, the more the organic additive arranged at the gas-liquid interface, the lower the surface tension of glycol aqueous (GA) solution and the higher the contamination degree. Furthermore, the foam characteristics and contamination degree of ACs are highly affected by the working conditions, such as airflow, operating temperature, and gas pressure. Besides, the defoaming rate can be accelerated by adding an antifoaming agent or ultrasonic processing; however, the defoaming effect of the natural static method and pressuring positively treatment is disappointing. To further improve the defoaming effect, several efficient synergetic methods of defoaming have been proposed.

Effect of Additives on the Foam Behavior of Aviation Coolants: Tendency, Stability, and Defoaming

The foam tendency of aviation coolants (ACs) can be greatly influenced by additives. This study investigates the effect of additives on foam behaviors based on four commercial ACs and glycol aqueous solutions added with different additives. Experimental results show that the foam tendency of ACs can be greatly influenced by surfactants; however, inorganic salts have little effect on foam tendency. The volume of generated foam reaches up to 350 mL after ventilation for an AC with a surfactant, much larger than 40 mL of an AC with an inorganic salt. The surface tension of ACs reduces with the addition of surfactants, the lower the surface tension, the more the foam formation. Furthermore, the presence of arranged surfactants at the gas-solution interface can increase the intermolecular forces and enhance the liquid and viscosity of film elasticity, thereby enhancing the foam stability. Besides, the surfactants would weaken the gas diffusion of foams and affect the defoaming property of ACs accordingly.

Encapsulation of Fe nanoparticles into an N-doped carbon nanotube/nanosheet integrated hierarchical architecture as an efficient and ultrastable electrocatalyst for the oxygen reduction reaction

The exploration of cost-effective, highly efficient and robust electrocatalysts toward the oxygen reduction reaction (ORR) is of paramount significance for the advancement of future renewable energy conversion devices, and yet still remains a great challenge. Herein, we demonstrate a straightforward one-step pyrolysis strategy for the scalable synthesis of an iron-nitrogen-carbon hierarchically nanostructured catalyst, in which Fe-based nanoparticles are encapsulated in bamboo-like N-doped carbon nanotubes in situ rooted from porous N-doped carbon nanosheets (Fe@N-C NT/NSs). The delicate fabrication of such an 0D/1D/2D integrated hierarchical architecture with encased Fe species and open configuration renders the formed Fe@N-C NT/NSs with sufficient confined active sites, reduced charge transfer resistance, improved diffusion kinetics and outstanding mechanical strength. As such, compared with commercial Pt/C, the optimized Fe@N-C NT/NSs catalyst exhibits efficient ORR activity, superior durability and strong tolerance to methanol in KOH medium. More impressively, when assembled as a cathode catalyst in a microbial fuel cell, the Fe@N-C NT/NSs electrode displays significantly enhanced power density and output voltage in comparison with commercial Pt/C, holding great promise in practical energy conversion devices. What's more, the simple yet reliable synthesis strategy developed here may shed light on the future design of advanced high-efficiency hierarchical architectures for diverse electrochemical applications and beyond.

Carbon-Based Nanocages: A New Platform for Advanced Energy Storage and Conversion

Energy storage and conversion play a crucial role in modern energy systems, and the exploration of advanced electrode materials is vital but challenging. Carbon-based nanocages consisting of sp² carbon shells feature a hollow interior cavity with sub-nanometer microchannels across the shells, high specific surface area with a defective outer surface, and tunable electronic structure, much different from the intensively studied nanocarbons such as carbon nanotubes and graphene. These structural and morphological characteristics make carbon-based nanocages a new platform for advanced energy storage and conversion. Up-to-date synthetic strategies of carbon-based nanocages, the utilization of their unique porous structure and morphology for the construction of composites with foreign active species, and their significant applications to the advanced energy storage and conversion are reviewed. Structure-performance correlations are discussed in depth to highlight the contribution of carbon-based nanocages. The research challenges and trends are also envisaged for deepening and extending the study and application of this multifunctional material.

Iron Nanoparticles Encapsulated in S,N-Codoped Carbon: Sulfur Doping Enriches Surface Electron Density and Enhances Electrocatalytic Activity toward Oxygen Reduction

Development of highly efficient nonprecious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media is challenging but of great significance. Herein, an effective ORR catalyst based on Fe nanoparticles encapsulated by S,N-codoped few-layer defective carbon (Fe@S,N-DC) was synthesized via a microwave-assisted strategy. The obtained Fe@S,N-DC nanocomposite showed a remarkable electrocatalytic activity toward ORR in acidic media, with a half-wave potential (E_1/2) of +0.785 V versus reversible hydrogen electrode, which was 80 mV more positive than that of the sulfur-free counterpart (Fe@N-DC). Furthermore, due to the protection by the S,N-codoped carbon shell, the Fe@S,N-DC nanocomposite displayed apparent stability with only a 13 mV negative shift of E_1/2 after 10,000 cycles and excellent tolerance to methanol. X-ray absorption near-edge spectroscopy measurements confirmed the formation of multiple defective sites on the S,N-codoped carbon surface and strong interfacial electron transfer from the Fe core to the outer carbon surface, as compared to the sulfur-free counterpart. The enriched electron density on the defective carbon surface of Fe@S,N-DC, induced by the interfacial electron transfer, facilitated the reduction of O₂ to OOH*, leading to enhanced ORR performance. These results shed light on the significance of S doping in Fe-N-C catalysts in the design of high-performance NPM catalysts for ORR in acidic media.

Bubble-Assisted Three-Dimensional Ensemble of Nanomotors for Improved Catalytic Performance

Combining catalysts with active colloidal matter could keep catalysts from aggregating, a major problem in chemical reactions. We report a kind of ensemble of bubble-cross-linked magnetic colloidal swarming nanomotors (B-MCS) with enhanced catalytic activity because of the local increase of the nanocatalyst concentration and three-dimensional (3D) fluid convection. Compared with the two-dimensional swarming collective without bubbles, the integral rotation was boosted because of the dynamic dewetting and increased slip length caused by the continuously ejected tiny bubbles. The bubbles cross-link the nanocatalysts and form stack along the vertical axis, generating the 3D network-like B-MCS ensemble with high dynamic stability and low drag resistance. The generated B-MCS ensemble exhibits controllable locomotion performance when applying a rotating magnetic field. Benefiting from locally increased catalyst concentration, good mobility, and 3D fluidic convection, the B-MCS ensemble offers a promising approach to heterogeneous catalysis.