Energy Applications of Magnetocaloric Materials

The need for energy‐efficient and environmentally friendly refrigeration, heat pumping, air conditioning, and thermal energy harvesting systems is currently more urgent than ever. Magnetocaloric energy conversion is among the best available alternatives for achieving these technological goals and has been the subject of substantial basic and applied research over the last two decades. The subject is strongly interdisciplinary, requiring proper understanding and efficient integration of knowledge in different specialized fields. This review article presents a historical and up‐to‐date account of the energy‐related applications of magnetocaloric materials and information about their processing and magnetic fields, thermodynamics, heat transfer, and other relevant characteristics. The article also discusses the conceptual design of magnetocaloric refrigeration and power generation systems and some guidelines for future research in the field.     

High Power Magnetic Field Energy Harvesting through Amplified Magneto‐Mechanical Vibration

Internet of Things (IoT) is driving the development of new generation of sensors, communication components, and power sources. Ideally, IoT sensors and communication components are expected to be powered by sustainable energy source freely available in the environment. Here, a breakthrough in this direction is provided by demonstrating high output power energy harvesting from very low amplitude stray magnetic fields, which exist everywhere, through magnetoelectric (ME) coupled magneto‐mechano‐electric (MME) energy conversion. ME coupled MME harvester comprised of multiple layers of amorphous magnetostrictive material, piezoelectric macrofiber composite, and magnetic tip mass, interacts with an external magnetic field to generate electrical energy. Comprehensive experimental investigation and a theoretical model reveal that both the magnetic torque generated through magnetic loading and amplification of magneto‐mechanical vibration by ME coupling contributes toward the generation of high electrical power from the stray magnetic field around power cables of common home appliances. The generated electrical power from the harvester is sufficient for operating microsensors (gyro, temperature, and humidity sensing) and wireless data transmission systems. These results will facilitate the deployment of IoT devices in emerging intelligent infrastructures.     

Itinerant Magnetism in Metallic CuFe2Ge2

Theoretical calculations are performed to understand the electronic structure and magnetic properties of CuFe₂Ge₂. The band structure reveals large electron density N(E_F) at the Fermi level suggesting a strong itinerant character of magnetism. The Fermi surface is dominated by two dimensional sheet like structures, with potentially strong nesting between them. The magnetic ground state appears to be ferromagnetic along a and antiferromagnetic in other directions. These results show that CuFe₂Ge₂ is an antiferromagnetic metal, with similarities to the Fe-based superconductors; such as magnetism with substantial itinerant character and coupling between magnetic order and electrons at the Fermi energy.     

Materials Challenges for High Performance Magnetocaloric Refrigeration Devices

Magnetocaloric materials with a Curie temperature near room temperature have attracted significant interest for some time due to their possible application for high‐efficiency refrigeration devices. This review focuses on a number of key issues of relevance for the characterization, performance and implementation of such materials in actual devices. The phenomenology and fundamental thermodynamics of magnetocaloric materials is discussed, as well as the hysteresis behavior often found in first‐order materials. A number of theoretical and experimental approaches and their implications are reviewed. The question of how to evaluate the suitability of a given material for use in a magnetocaloric device is covered in some detail, including a critical assessment of a number of common performance metrics. Of particular interest is which non‐magnetocaloric properties need to be considered in this connection. An overview of several important materials classes is given before considering the performance of materials in actual devices. Finally, an outlook on further developments is presented.     

Opening Two‐Dimensional Materials for Energy Conversion and Storage: A Concept

The development of two‐dimensional (2D) materials is experiencing a renaissance since the adventure of graphene. 2D materials typically exhibit strong in‐plane covalent bonding and weak out‐of‐plane van der Waals interactions through the interlayer gap. Opening 2D materials is an effective way to alter the physical and chemical properties, such as band gap, conductivity, optical property, thermoelectric property, photovoltaic property and superconductivity. A larger interlayer distance means more accessible active sites for catalysis, an ion‐accessible surface in the interlayer space, which may greatly enhance the performance of 2D materials for energy conversion and storage. Moreover, opening 2D materials by intercalation can change the band filling state and the Fermi level. This review mainly focuses on the opening of 2D materials and their subsequent applications in energy conversion and storage fields, expecting to promote the development of such a new class of materials, namely expanded 2D materials. The exciting progresses of these expanded materials made in both energy conversion and storage devices including solar cells, thermoelectric devices, electrocatalyst, supercapacitors and rechargeable batteries, is presented and discussed in depth. Furthermore, prospects and further developments in these exciting fields of the expanded 2D materials are also commented.     

Visualization of RNA crystal growth by atomic force microscopy.

The crystallization of transfer RNA (tRNA) was investigated using atomic force microscopy (AFM) over the temperature range from 4 to 16 degrees C, and this produced the first in situ AFM images of developing nucleic acid crystals. The growth of the (110) face of hexagonal yeast tRNAPhe crystals was observed to occur at steps on vicinal hillocks generated by multiple screw dislocation sources in the temperature range of 13.5-16 degrees C. Two-dimensional nucleation begins to dominate at 13.5 degrees C, with the appearance of three-dimensional nuclei at 12 degrees C. The changes in growth mechanisms are correlated with variations in supersaturation which is higher in the low temperature range. Growth of tRNA crystals was characterized by a strong anisotropy in the tangential step movement and transformation of growth modes on single crystals were directly observed by AFM over the narrow temperature range utilized. Finally, lattice resolution images of the molecular structure of surface layers were recorded. The implications of the strong temperature dependence of tRNAPhe crystal growth are discussed in view of improving and better controlling crystallization of nucleic acids.     

Dynamics of the First‐Order Metamagnetic Transition in Magnetocaloric La(Fe,Si)13: Reducing Hysteresis

The influence of dynamics and sample shape on the magnetic hysteresis in first‐order magnetocaloric metamagnetic LaFe_13–xSi_x with x = 1.4 is studied. In solid‐state magnetic cooling, reducing magnetic and thermal hysteresis is critical for refrigeration cycle efficiency. From magnetization measurements, it is found that the fast field‐rate dependence of the hysteresis can be attributed to extrinsic heating directly related to the thickness of the sample and the thermal contact with the bath. If the field is paused partway through the transition, the subsequent relaxation is strongly dependent on shape due to both demagnetizing fields and thermal equilibration; magnetic coupling between adjacent sample fragments can also be significant. Judicious shaping of the sample can both increase the onset field of the ferromagnetic–paramagnetic (FM–PM) transition but have little effect on the PM–FM onset, suggesting a route to engineer the hysteresis width by appropriate design. In the field‐paused state, the relaxation from one phase to the other slows with increasing temperature, implying that the process is neither thermally activated or athermal; comparison with the temperature dependence of the latent heat strongly suggests that the dynamics reflect the intrinsic free energy difference between the two phases.     

Group-theoretical approaches in molecular magnetism: Metal clusters

In this article I review the application of the group-theoretical approaches to a wide area of molecular magnetism dealing with metal clusters. The following main aspects are discussed: (1) irreducible tensor operator (ITO) approach that is based on the so-called “spin-symmetry”. Use of this approach in molecular magnetism has given a revolutionary impact on the evaluation of the energy levels, thermodynamic and spectroscopic properties of high-nuclearity metal clusters. ITO approach facilitated development and applications of the isotropic and anisotropic spin-Hamiltonians and the study of the magnetic anisotropy in clusters containing orbitally degenerate metal ions; (2) group-theoretical classification (assignment) of the exchange multiplets based on both spin-symmetry and point symmetry that allows to analyze the non-Heisenberg forms of the exchange interaction and magnetic anisotropy in general terms, establishes selection rules for magnetic resonance transitions and facilitates computation of spin levels. This approach allows also to reveal the selection rules for the active Jahn–Teller coupling and to clear understand the interrelation between spin frustration and structural instabilities; (3) group-theoretical classification of the delocalized electronic and electron-vibrational states of mixed-valence compounds in terms of spin and point symmetries (including delocalization of the electronic pair) that essentially reduces the time of calculations and provides direct assess to the selection rules for different kinds of transitions. This becomes crucial in the dynamical vibronic problems inherently related to mixed-valency even for the truncated basis sets when the calculations become hardly executable not only in the case of strong vibronic coupling but even provided that the vibronic coupling is moderate. The proposed approach includes the design of the symmetry adapted vibronic basis and can enormously extend computational abilities in the dynamical problem of mixed-valency.     

Evidence for monomeric bacteriochlorophyll in P800 of the photoreaction center from Rhodospirillum rubrum.

To find out whether weak or strong coupling exists between the bacteriochlorophyll molecules of the photoreaction center, the relative efficiency of energy transfer to P870 was measured at 795 nm and at 808 nm, at room temperature and at 77 degrees K. At room temperature, both relative efficiencies are close to 100%. However, at 77 degrees K, 795 nm light has a quantum efficiency of 76% and 808 nm light has an efficiency of 87%. These results confirm the fact that P800 is formed of at least one short wavelength component and one long wavelength component. Moreover, the short wavelength component is weakly coupled to both P870 and to the long wavelength component of P800. The conclusion is that the short wavelength component is due to monomeric bacteriochlorophyll. By comparison with other data, all four bacteriochlorophyll molecules of the photoreaction center are inferred to be monomeric.     

Catalytic pyrolysis of Miscanthus x giganteus over activated alumina

The catalytic effects of activated alumina (Al(2)O(3)) on the pyrolysis of Miscanthusxgiganteus, a new energy crop, were investigated. Catalytic pyrolysis experiments carried out under static and nitrogen atmospheres were performed in a fixed-bed reactor. The final pyrolysis temperature was kept constant at 550 degrees C in all of the experiments. The effect of catalyst loading (by weight of feedstock as 10%, 20%, 40%, 60%, 80% and 100%), heating rate (10 degrees C and 50 degrees Cmin(-1)), nitrogen flow rate (50, 100, 150 and 200cm(3)min(-1)) on the pyrolysis conversion and product yields were investigated. The results were compared with those obtained in non-catalytic pyrolysis. Activated alumina catalyst has a strong influence on the Miscanthusxgiganteus pyrolysis product and conversion yield. Furthermore, the catalytic bio-oils obtained from catalytic pyrolysis under static and nitrogen atmospheres were examined using elemental analysis, column chromatography, Fourier transform infrared (FTIR) and nuclear magnetic resonance ((1)H NMR) spectroscopy methods.     

Improved Processability and Efficiency of Colloidal Quantum Dot Solar Cells Based on Organic Hole Transport Layers

High‐efficiency solid‐state‐ligand‐exchange (SSE) step‐free colloidal quantum dot photovoltaic (CQDPV) devices are developed by employing CQD ink based active layers and organic (Polythieno[3,4‐b]‐thiophene‐co‐benzodithiophene (PTB7) and poly(3‐hexylthiophene) (P3HT)) based hole transport layers (HTLs). The device using PTB7 as an HTL exhibits superior performance to that using the current leading organic HTL, P3HT, because of favorable energy levels, higher hole mobility, and facilitated interfacial charge transfer. The PTB7 based device achieves power conversion efficiency (PCE) of 9.60%, which is the highest among reported CQDPVs using organic HTLs. This result is also comparable to the PCE of an optimized device based on a thiol‐exchanged p‐type CQD, the current‐state‐of‐the‐art HTL. From the viewpoint of device processing, the fabrication of CQDPVs is achieved by direct single‐coating of CQD active layers and organic HTLs at low temperature without SSE steps. The experimental results and device simulation results in this work suggest that further engineering of organic HTL materials can open new doors to improve the performance and processing of CQDPVs.     

Recent Advances and Emerging Trends in Photo‐Electrochemical Solar Energy Conversion

Photo‐electrochemical (PEC) solar energy conversion offers the promise of low‐cost renewable fuel generation from abundant sunlight and water. In this Review, recent developments in photo‐electrochemical water splitting are discussed with respect to this promise. State‐of‐the‐art photo‐electrochemical device performance is put in context with the current understanding of the necessary requirements for cost‐effective solar hydrogen generation (in terms of solar‐to‐hydrogen conversion efficiency and system durability, in particular). Several important studies of photo‐electrochemical hydrogen generation at p‐type photocathodes are highlighted, mostly with protection layers (for enhanced durability), but also a few recent examples where protective layers are not needed. Recent work with the widely studied n‐type BiVO₄ photoanode is detailed, which highlights the needs and necessities for the next big photoanode material yet to be discovered. The emerging new research direction of photo‐electrocatalytic upgrading of biomass substrates toward value‐added chemicals is then discussed, before closing with a commentary on how research on PEC materials remains a worthwhile endeavor.     

Phenomenological and molecular models of biological proton transfers

In this paper, the main models describing the transfer of a proton in a molecular system are presented. Models valid when the intersite coupling is weak (non-adiabatic and electronically adiabatic regimes) and strong (adiabatic regime) are described. We distinguish molecular models in which the rate constant is obtained by considering explicitly various degrees of freedom of the system and simpler, phenomenological models built to account for the kinetic isotope effect. The relations between the various models are discussed. Their application to specific systems is illustrated by several studies reported in the literature, with a special emphasis on biological systems.     

Novel 2D Sb2S3 Nanosheet/CNT Coupling Layer for Exceptional Polysulfide Recycling Performance

2D layer‐structured materials are considered a promising candidate as a coupling material in lithium sulfur batteries (LSBs) due to their high surface‐volume ratio and abundant active binding sites, which can efficiently mitigate shuttling of soluble polysulfides. Herein, an electrochemical Li intercalation and exfoliation strategy is used to prepare 2D Sb₂S₃ nanosheets (SSNSs), which are incorporated onto a separator in LSBs as a new 2D coupling material for the first time. The cells containing a rationally designed separator which is coated with an SSNS/carbon nanotube (CNT) coupling layer deliver a much improved specific capacity with a remarkable 0.05% decay rate for over 200 cycles at a current density of 2 C. The capability of the SSNSs to entrap polysulfides through their favorable interfacial functionality and the high electrical conductivity of the CNT network facilitates recycling of active materials. The first‐principle calculations verify the important roles of SSNSs, which demonstrate ideal binding strengths (1.33–2.14 eV) to entrap Li₂S_x as well as a low‐energy barrier (189 meV) for Li diffusion. These findings offer new insights into discovering novel coupling layers for high‐performance LSBs and shed new light on the application of 2D layer‐structured materials in energy storage systems.     

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.     

Production of reactive oxygen species by isolated mitochondria of the Antarctic bivalve Laternula elliptica (King and Broderip) under heat stress

Formation of reactive oxygen species (ROS) in mitochondrial isolates from gill tissues of the Antarctic polar bivalve Laternula elliptica was measured fluorimetrically under in vitro conditions. When compared to the rates measured at habitat temperature (1 degrees C), significantly elevated ROS formation was found under temperature stress of 7 degrees C and higher. ROS formation correlated significantly with oxygen consumption in individual mitochondrial preparations over the entire range of experimental temperatures (1-12 degrees C). ROS generation per mg of mitochondrial protein was significantly higher in state 3 at maximal respiration and coupling to energy conservation, than in state 4+, where ATPase-activity is inhibited by oligomycin and only proton leakage is driving the residual oxygen consumption. The percent conversion of oxygen to the membrane permeant hydrogen peroxide amounted to 3.7% (state 3) and 6.5% (state 4+) at habitat temperature (1 degrees C), and to 7% (state 3) and 7.6% (state 4+) under experimental warming to 7 degrees C. This is high compared to 1-3% oxygen to ROS conversion in mammalian mitochondrial isolates and speaks for a comparatively low control of toxic oxygen formation in mitochondria of the polar bivalve. However, low metabolic rates at cold Antarctic temperatures keep absolute rates of mitochondrial ROS production low and control oxidative stress at habitat temperatures. Mitochondrial coupling started to fall beyond 3 degrees C, closely to pejus temperature (4 degrees C) of the bivalve. Accordingly, the proportion of state 4 respiration increased from below 30% at 1 degrees C to over 50% of total oxygen consumption at 7 degrees C, entailing reduced ADP/O ratios under experimental warming. Progressive mitochondrial uncoupling and formation of hazardous ROS contribute to bias mitochondrial functioning under temperature stress in vitro. Deduced from a pejus temperature, heat stress commences already at 5 degrees C, and is linked to progressive loss of phosphorylation efficiency, increased mitochondrial oxygen demand and elevated oxidative stress above pejus temperatures.     

Magnetic susceptibility of laccases and ceruloplasmin.

1. Recent magnetic susceptibility measurements on laccase (monophenol,dihydroxyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1) from the lacquer tree Rhus vernicifera showed a deviation from Curie behaviour above 50 K, which was taken as evidence for an antiferromagnetically coupled Cu(II)-Cu(II) pair in the oxidized enzyme. The magnetic susceptibility of this protein has been reinvestigated. Further measurements on laccase from the fungus Polyporus versicolor and human ceruloplasmin (iron(II):oxygen oxidoreductase, EC 1.16.3.1) are presented. 2. The magnetic susceptibility of fungal laccase and lacquer tree laccase can be accounted for by the EPR detectable copper ions in the temperature range 40--300 K. 3. If an antiferromagnetically coupled Cu(II)-Cu(II) pair exists in the laccases, then the coupling, expressed as --J, should be at least of the order of 300 cm-1, as deduced from the Curie dependence of the susceptibility and the sensitivity in our measurements. 4. If an analogy with the laccases is assumed for the EPR invisible copper in ceruloplasmin then a limiting value of the coupling may be deduced also in this case, with --J at least of the order of 200 cm-1.     

The Resource Basis of Magnetic Refrigeration

Emerging economies such as China and India are currently experiencing a “refrigeration revolution.” Energy spent for domestic cooling is expected to outreach that for heating worldwide over the course of the twenty‐first century. Magnetic refrigeration is an alternative cooling technology that works without gas‐based refrigerants and has the potential to be significantly more energy efficient. We evaluate to what extent the raw materials needed to produce this kind of technology on a mass‐market scale are critical in terms of demand and supply, thus identifying potential supply bottlenecks that might hinder the breakthrough of this promising technology. We assess the criticality of three promising magnetocaloric materials, that is, Gd₅(SiGe)₄, La(FeSi)₁₃, and (MnFe)₂P), as well as of Nd₂Fe₁₄B, as the candidate permanent magnet material to drive the cooling cycle. The Gd‐based alloys are disqualified as a mass‐market refrigerant in terms of resource criticality, whereas La‐ and Mn‐based alloys are much less problematic. Given the current state of technology and projected resource supply, Nd in Nd₂Fe₁₄B magnets would experience a significant bottleneck only at a later innovation stage, that is, when magnetic cooling technology would largely dominate the domestic refrigerator and air‐conditioning market.     

High‐Performance Thermomagnetic Generators Based on Heusler Alloy Films

Recent developments on Heusler alloys including Ni–Mn–X and Ni–Co–Mn–X (X = Ga, In, Sn,…) demonstrate multiferroic phase transformations with large abrupt changes in lattice parameters of several percent and corresponding abrupt changes in ferromagnetic ordering near the transition temperatures. These materials enable a new generation of thermomagnetic generators that convert heat to electricity within a small temperature difference below 5 K. While thermodynamic calculations on this energy conversion method predict a power density normalized to material volume of up to 300 mW cm^?3, experimental results have been in the range of µW cm^?3. Challenges are related to the development of materials with bulk‐like single‐crystal properties as well as geometries with large surface‐to‐volume ratio for rapid heat exchange. This study demonstrates efficient thermomagnetic generation via resonant actuation of freely movable thin‐film devices of the Heusler alloy Ni–Mn–Ga with unprecedented power density of 118 mW cm^?3 that compares favorably with the best thermoelectric generators. Due to the large temperature‐dependent change of magnetization of the films, a periodic temperature change of only 3 K is required for operation. The duration of thermomagnetic duty cycle is only about 12 ms, which matches with the period of oscillatory motion.