Probing Charge Transport and Thermoelectricity in Molecular Junctions

The study of charge transport and thermoelectricity in molecular junctions is of fundamental interest for understanding charge transport mechanisms and provides knowledge critical for the development of nanotechnologies including electronics, energy conversion and thermal management. In spite of a large amount of theoretical and experimental work into charge transport and thermoelectric properties of various molecular junctions, several important questions remain unsolved. Quantum phenomena dominate transport in molecular junctions, therefore, a natural question to ask is whether it is possible to tune the thermoelectric properties of molecular junctions via quantum interferences. To answer this question, I present work where we investigated charge and thermoelectric properties in molecular junctions based on two oligo (phenylene ethynylene) (OPE) derivatives where quantum interference effects are expected to arise. Our experiments reveal that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ~20 μV/K) compared with para-OPE3 (with ~10 μV/K). Results from both single-molecule junction and monolayer experiments correspond well with each other and agree well with computational predictions made by our collaborators. Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions. Along with enhancing thermoelectric properties of molecular junctions via quantum interference, past theoretical work has proposed another strategy to tune the thermoelectric properties in molecular junctions by varying the metal centres incorporated in porphyrins. The tunability of thermoelectric properties in these junctions, however, have not been experimentally explored. To explore the feasibility of tuning the thermoelectric properties in these junctions, we conducted measurements in Au-porphyrin-Au, Au-(Cu-porphyrin)-Au and Au-(Zn-porphyrin)-Au junctions. To achieve better thermoelectric performance, we replaced the thiol end groups that are typically employed in a series of metallo-porphyrins with triisopropylsilyl end groups, which enable a direct C-Au bond resulting in an increase of the electrical conductance. In fact, our single molecule experiments show nearly two orders of magnitude increase in the electrical conductance of junctions compared to previous work that employed thiol end groups. The thermoelectric experiments reveal tunable thermopower through molecules with different metal centres. Overall, among the molecules studied in our work, we find that Au-(Zn-porphyrin)-Au junctions exhibit the best thermoelectric performance. In addition to the energy conversion of heat-to-electricity as discussed above, thermoelectric effects are also expected to enable cooling in molecular junctions through Peltier effects and in principle such Peltier cooling in molecular junctions may be applied to fabricate refrigerators at molecular scale. However, experimental observation of Peltier cooling in molecular junctions has not been possible. Here, I discuss the experimental observation of Peltier cooling in molecular junctions. The molecular junctions studied here are Au-(biphenyl-4,4'-dithiol)-Au, Au-(terphenyl-4,4''-dithiol)-Au and Au-(4,4'-bipyridine)-Au, of which the charge transport and thermoelectric properties are widely studied in the field. Our results unambiguously show cooling in molecular junctions under small bias voltage and reveal the relationship between heating or cooling and charge transport mechanisms in studied molecular junctions. Our experimental results are supported by computational results from our collaborators.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149871/1/mruijiao_1.pd

Electrical conductance and thermopower of β-substituted porphyrin molecular junctions ─ synthesis and transport

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn–P1 and Zn–P2. Past theory suggests that P1 and Zn–P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn–P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn–P1 junctions is relatively close, and the same holds for P2 and Zn–P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn–P1 junctions show the largest thermopower and Zn–P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn–P1 junctions is robustly predicted to be larger than those of P2 and Zn–P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn–P1 and Zn–P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings

Study of Top Dead Center Measurement and Correction Method in a Diesel Engine

Abstract: The thermal loss angle error analysis and maximum pressure determination method analysis were conducted first. Then the polytropic exponent method, the inflection point analysis, the loss function method and the symmetry method were utilized under different rotating speed, load and cooling water temperature, to calculate TDC in D6114 diesel engine and the results were compared with TDC position measured under the same condition with direct method of measurement. The study proved that (1) thermal loss angle of the diesel engine ranges from -1.0 ~ -0.6°CA; (2) Thermal loss angle is mainly affected by rotating speed and is reducing when rotate speed increases;(3) the symmetry method is generally the optimum for calculating the thermal loss angle of automotive diesel engines, with an error within 0.2°CA

A non-volatile thermal switch for building energy savings

Compared with traditional static insulation, a thermally switchable building envelope could reduce annual heating and cooling loads by intermittently coupling to the outside environment when beneficial. Here, we demonstrate a voltage-actuated, contact/non-contact thermal switch that meets the unique challenges of this application. The switch is non-volatile, consuming electricity only briefly while switching and none to hold steady state. The switch ratio is 12, the off state has a low effective thermal conductivity of 0.045Wm-1K-1, comparable to fiberglass insulation, and the performance is stable over 1,000 switching cycles. Numerical simulations using real-world climate data show that combining this thermal switch with a thermal storage layer in a building envelope can yield annual energy savings of 9%–55% (heating) and 17%–76% (air conditioning), depending on the climate zone. The greatest benefits are realized when the exterior temperature crosses well above and below the desired interior temperature within a single 24 h period

Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring (para-OPE3) and meta-connected central ring (meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions

Extreme fast charging of batteries using thermal switching and self-heating

The long charge time of electric vehicles compared with the refueling time of gasoline vehicles, has been a major barrier to the mass adoption of EVs. Currently, the charge time to 80% state of charge in electric vehicles such as Tesla with fast charging capabilities is >30 minutes. For a comparable recharging experience as gasoline vehicles, governments and automobile companies have set <15 min with 500 cycles as the goal for extreme fast charging (XFC) of electric vehicles. One of the biggest challenges to enable XFC for lithium-ion batteries (LIBs) is to avoid lithium plating. Although significant research is taking place to enable XFC, no promising technology/strategy has still emerged for mainstream commercial LIBs. Here, we propose a thermally modulated charging protocol (TMCP) by active thermal switching for XFC, i.e., retaining the battery heat during XFC with the switch OFF for boosting the kinetics to avoid lithium plating while dissipating the heat after XFC with the switch ON for reducing side reactions. Our proposed TMCP strategy enables XFC of commercial high-energy-density LIBs with charge time 500 cycles while simultaneously beating other targets set by US Department of energy (discharge energy density > 180 Wh/kg and capacity loss < 4.5%). Further, we develop a thermal switch based on shape memory alloy and demonstrate the feasibility of integrating our TMCP in commercial battery thermal management system

Extreme fast charging of commercial Li-ion batteries via combined thermal switching and self-heating approaches

Abstract The mass adoption of electric vehicles is hindered by the inadequate extreme fast charging (XFC) performance (i.e., less than 15 min charging time to reach 80% state of charge) of commercial high-specific-energy (i.e., >200 Wh/kg) lithium-ion batteries (LIBs). Here, to enable the XFC of commercial LIBs, we propose the regulation of the battery’s self-generated heat via active thermal switching. We demonstrate that retaining the heat during XFC with the switch OFF boosts the cell’s kinetics while dissipating the heat after XFC with the switch ON reduces detrimental reactions in the battery. Without modifying cell materials or structures, the proposed XFC approach enables reliable battery operation by applying <15 min of charge and 1 h of discharge. These results are almost identical regarding operativity for the same battery type tested applying a 1 h of charge and 1 h of discharge, thus, meeting the XFC targets set by the United States Department of Energy. Finally, we also demonstrate the feasibility of integrating the XFC approach in a commercial battery thermal management system

Extreme fast charging of commercial Li-ion batteries via combined thermal switching and self-heating approaches

The mass adoption of electric vehicles is hindered by the inadequate extreme fast charging (XFC) performance (i.e., less than 15 min charging time to reach 80% state of charge) of commercial high-specific-energy (i.e., &gt;200 Wh/kg) lithium-ion batteries (LIBs). Here, to enable the XFC of commercial LIBs, we propose the regulation of the battery's self-generated heat via active thermal switching. We demonstrate that retaining the heat during XFC with the switch OFF boosts the cell's kinetics while dissipating the heat after XFC with the switch ON reduces detrimental reactions in the battery. Without modifying cell materials or structures, the proposed XFC approach enables reliable battery operation by applying &lt;15 min of charge and 1 h of discharge. These results are almost identical regarding operativity for the same battery type tested applying a 1 h of charge and 1 h of discharge, thus, meeting the XFC targets set by the United States Department of Energy. Finally, we also demonstrate the feasibility of integrating the XFC approach in a commercial battery thermal management system