The model of a three-terminal thermoelectric refrigerator with ideal tunneling quantum dots is established. It consists of a cavity connected to two quantum dots embedded between two electron reservoirs at different temperatures and chemical potentials. According to the Landauer formula the expressions for the heat current, the cooling rate and the coefficient of performance (COP) are derived analytically. The performance characteristic curves of the cooling rate versus the coefficient of performance are plotted with numerical calculation. The optimal regions of the cooling rate and the COP are determined. Moreover, we optimize the cooling rate and the COP with respect to the position of energy level of the right quantum dot, respectively. The influence of the width of energy level and the temperature ratio on performance of the three-terminal thermoelectric refrigerator is analyzed. Lastly, when the width of energy level is small enough, the optimal performance of the refrigerator is discussed in detail.
Abstract Dielectric film capacitors have aroused considerable attention on account of the fast development of pulsed power systems. However, enhanced energy density is always acquired at the cost of deteriorated charge/discharge efficiency. Herein, well balanced energy density and efficiency are achieved in a series of reasonably designed bilayer composites consisting of a ferroelectric layer and a paraelectric layer at the meantime. It is interesting to find that, when merely 1.6 wt% Zr(HPO 4 ) 2 nanosheets are introduced into the ferroelectric layer, a substantially improved energy density of 11.22 J cm −3 , which is about 165% that of the bilayer composite without Zr(HPO 4 ) 2 nanosheets, is achieved at 650 kV mm −1 . Meanwhile, a high charge/discharge efficiency of 89.8% and a low loss tangent of 0.024@10 kHz which is much lower than the pristine ferroelectric polymer layer (0.058@10 kHz) is maintained. Furthermore, finite element simulation reveals that the electric breakdown paths will develop along the macroscopical‐interfaces between adjoining layers and the microcosmic‐interfaces between the Zr(HPO 4 ) 2 nanosheets and polymer matrix, which can effectively increase the length of breakdown paths and contribute to improved breakdown strength. This work demonstrates that the Zr(HPO 4 ) 2 nanosheets can be promising fillers for other high‐performance dielectric composites.
Controllable designing of heteroatom-doped carbon catalysts provides an insightful strategy for boosting the performance and kinetics of the oxygen reduction/evolution reaction (ORR/OER). However, the role of oxygen species is usually omitted. Herein, a facile oxygen engineering strategy is proposed to tune the oxygen species in N-doped porous carbon nanofibers (NPCNFs-O) via a facile electrospinning method, in which β-cyclodextrin acts as the pore inducer and oxygen regulator. Benefitting from the large specific surface area and synergistic effect of N,O codoping, the NPCNF-O catalyst exhibits superior ORR (E1/2 = 0.85 V vs reversible hydrogen electrode (RHE)) and OER (Ej = 10 = 1.556 V vs RHE) activities with excellent stability. Both experimental and theoretical calculations verify the crucial role of carboxyl groups, which regulate the local charge density and reduce the reaction energy barrier for enhancing the oxygen electrocatalytic activity. Moreover, a rechargeable zinc–air battery using NPCNF-O as the air cathode demonstrates a maximum power density of 125.1 mW cm–2 and long-term durability. Importantly, NPCNF-O can be served as an integrated freestanding electrode for portable zinc–air batteries. The work brings brilliant fundamental insights for constructing efficient metal-free carbon material catalysts for future energy conversion and storage systems.
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