Boron-Doped Activated Carbon Supports for Cobalt-Catalyzed Oxygen Evolution in Alkaline Electrolyte.

Activated carbons (ACs) are the most widely used and attractive support materials for electrocatalytic applications because of their significant surface areas, high electrical conductivities, and moderate affinities toward supported metal catalysts. However, the corrosive behavior of ACs at oxidative potentials causes an inevitable reduction in the active surface area of supported catalysts, resulting in the continuous deterioration of their electrocatalytic performance. Therefore, the introduction of corrosion-resistant durable catalyst supports is essential for sustainable and efficient electrocatalysis. Here, we modified ACs to obtain different boron (B)-doped structures via doping-temperature controls to investigate the corrosion resistance of B-doped ACs. With increasing doping temperature, the B-doped ACs exhibited a decreased defect density and enhanced crystallinity owing to the accelerating dopant-induced graphitization. We found that the substitution of B atoms into the carbon lattice improved the structural integrity of the carbon structure, and cyclic voltammetry (CV) tests suggested that the highly B-substituted structures caused electrochemical surface passivation against carbon corrosion. Moreover, B-doped ACs significantly contributed to the increase in loading mass of cobalt (Co)-based catalyst on them and the electrochemical durability toward the oxygen evolution reaction as catalyst-support hybrid. The B22 (B-doped AC obtained at a 2200 °C B-doping temperature)-supported Co catalyst with the lowest oxidation current exhibited a voltage change of 32 mV at a current density of 10 mA/cm2 (ΔEj=10) after 10,000 cycles, which was a factor of ∼7 higher cycle durability and stability than that of the conventional IrO2 catalyst (ΔEj=10 = 205 mV). Here, we propose that surface engineering by B-doping to improve the structural integrity of ACs is an attractive method for designing durable electrocatalytic support materials.

Ammonium Ionic Liquid-Functionalized Phenothiazine as a New Redox Mediator for High Chemical Stability on the Anode Surface in Lithium-Air Batteries.

The application of redox mediators (RMs) as soluble catalysts can address the problem of insufficient contact between conventional solid catalysts for lithium-air batteries (LABs). However, oxidized RM molecules migrate to the lithium anode and react with lithium, which results in the accumulation of surface corrosion products that weaken the redox activity of the RM. This paper presents a new combination of phenothiazine (PTZ) as an RM and an ammonium-based ionic liquid (IL) source as a protective agent to prevent the side reactions with lithium and to enhance the electrochemical performance of LABs. IL-functionalized PTZ (IL-PTZ) was successfully synthesized through N-alkylation, quaternization, and anion-exchange reactions. IL-PTZ improved the chemical stability of the RM molecules on the lithium surface as well as the electrochemical performance. A microstructural analysis revealed that the IL group in the IL-PTZ molecules facilitated smooth lithium stripping/plating by blocking the side reactions between the RM and lithium. Compared with the LAB with the PTZ electrolyte, that with the IL-PTZ electrolyte exhibited a significantly higher discharge capacity (2500 mA h/g vs 1500 mA h/g) and a cycle life that was 2 times longer. The IL-PTZ molecule was demonstrated to exhibit great potential as a novel soluble catalyst for application in high-performance LABs.

Laser-Assisted Interface Engineering for Functional Interfacial Layer of Al/ZnO/Al Resistive Random Access Memory (RRAM).

In oxide-based RRAMs using reactive electrodes such as Al, the properties of spontaneously formed interfacial layers are critical factors in determining the resistive switching (RS) performance and reliability. This interfacial layer can provide the beneficial function of oxygen reservoir and series resistance, but is very labile and prone to deterioration, causing fatal reliability problems. Moreover, there are technical difficulties in manipulating and improving the functional interfacial layer due to the various interaction dynamics near the interface and the unstable thermodynamic properties of Al. In this work, laser-assisted interface engineering, which allows exquisite manipulation of the labile interfacial layer, is proposed to improve the reliability and performance of Al/ZnO/Al RRAMs. In addition to photothermal and photochemical effects, the proposed laser process enables fine control over out-diffusions of Al atoms in the vicinity of the ZnO/Al interface, forming a robust interfacial layer with a uniform morphology and abundant oxygen Frenkel pairs. This laser-engineered interfacial layer increases the RHRS/RLRS ratio by over 100-fold and reduces RHRS variation with improved oxygen reservoir ability. It also appears to reduce leakage current and power consumption by acting as a stable series resistance. The correlation between structural and stoichiometric properties of the functional interfacial layer and the performance and reliability of the RRAM is explicated. The results suggest that laser-assisted interface engineering can be one of the most promising methods to implement highly reliable, high-performance Al/ZnO/Al RRAMs.

Thermal degradation of refractory layered metamaterial for thermophotovoltaic emitter under high vacuum condition.

Emissivity-tunable metamaterials of layered refractory metal and dielectric have great potentials as a simple thermophotovoltaic (TPV) selective emitter due to its near-omnidirectional, polarization-independent, and broadband selective emissivity. However, it is known that the stability of the layered structure is limited by the oxidation of metals. While there still exists ambiguity concerning the main source of oxygen between adjacent oxide layers and external residual oxygen, most reports focus on the adjacent layers. In this report, thermal stability of a tungsten-based layered metamaterial is investigated under a high-vacuum environment with great care to reduce residual oxygen. The results show unprecedented thermal stability up to 1200 °C for 3 h without any measurable oxidation of metal. This implies that the interlayer diffusion of oxygen from adjacent oxide layers is not exclusively responsible for the oxidation of metal. At such a high temperature, the layered metamaterial theoretically yields a high convertible radiative power density of 3.04 W/cm2 with comparable spectral efficiency of 40.2%. Finally, after performing series of thermal tests under higher thermal loads, we propose a novel high-temperature degradation model for layered metamaterials, the stability of which is ultimately limited by the agglomeration of thin metal layers.

Thermal modeling and measurement of junctions in photovoltaic packages.

This study reports the direct thermal observation of the junction temperature and determination of the thermal resistance in a commercial solar cell package under actual operating conditions. A thermal transient method was the key method that was utilized for the thermal characterization of the device. Sunlight directed into the solar cell package was found to increase significantly the heat generation inside the solar cell package. It was shown that the operation of the solar cell package with sunlight of 1 sun resulted in a junction temperature of about 113 degrees C. The simulation data were in good agreement with the measured values. Detailed thermal performance of the solar cell package was projected using a Computational Fluid Dynamics (CFD) method under various operating conditions. Thermal resistance was determined under various operating conditions.