We are pleased to introduce the collection, 2021 Retrospective: Energy Materials. This collection showcases well-received spontaneous articles from the past couple of years, which have been specially handpicked by our Chief Editor, Prof. Liming Dai. The work presented here highlights the broad diversity of research performed across the section, and aims to put a spotlight on the main areas of interest. All research presented here displays strong advances in theory, experiment and methodology with applications to compelling problems. This collection aims to further support Frontiers’ strong community by recognizing highly deserving authors.

University of New South Wales

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Frontiers in Materials

Energy Materials

2021 Retrospective: Energy Materials

Solar light absorber surface is probably one of the most important components in solar still that dictates the distillate yield. In this work, a systematic study is conducted to investigate the effect of particle size and concentration of titanium oxide (TiO2) in black paint in increasing the solar still absorber surface temperature. The various available particle sizes, i.e., 20, 150, and 400 nm, are mixed in black paint with varying concentrations and are applied on the absorber plate. XRD is used for phase identification of as-received powders. UV-Vis spectroscopy is used to examine light absorption properties. Finally, extensive indoor testing (using an improvised solar simulator) and outdoor testing are conducted to optimize the concentration. An increase in surface temperature is observed with the introduction of TiO2 nanoparticles in black paint. Furthermore, the increase in particle size leads to an increase in temperature. The highest surface temperatures of 104.86°C, 105.42°C, and 106.32°C are recorded for specimens with particles sizes 20 nm (at 15 wt% concentration), 150 nm (at 10 wt% concentration), and 400 nm (at 7 wt% concentration), respectively. Furthermore, the highest temperature of 69.69°C is recorded for TiO2-400 nm specimens under outdoor conditions, which is 15.97% higher than that of the bare aluminum plate. The increase in surface temperature may be due to high UV absorption. Moreover, an increase in particle size leads to high light-scattering ability, further improving the light-harvesting ability.

Solar light absorber surface is probably one of the most important components in solar still that dictates the distillate yield. In this work, a systematic study is conducted to investigate the effect of particle size and concentration of titanium oxide (TiO2) in black paint in increasing the solar still absorber surface temperature. The various available particle sizes, i.e., 20, 150, and 400&nbsp;nm, are mixed in black paint with varying concentrations and are applied on the absorber plate. XRD is used for phase identification of as-received powders. UV-Vis spectroscopy is used to examine light absorption properties. Finally, extensive indoor testing (using an improvised solar simulator) and outdoor testing are conducted to optimize the concentration. An increase in surface temperature is observed with the introduction of TiO2 nanoparticles in black paint. Furthermore, the increase in particle size leads to an increase in temperature. The highest surface temperatures of 104.86°C, 105.42°C, and 106.32°C are recorded for specimens with particles sizes 20&nbsp;nm (at 15&nbsp;wt% concentration), 150&nbsp;nm (at 10&nbsp;wt% concentration), and 400&nbsp;nm (at 7&nbsp;wt% concentration), respectively. Furthermore, the highest temperature of 69.69°C is recorded for TiO2-400&nbsp;nm specimens under outdoor conditions, which is 15.97% higher than that of the bare aluminum plate. The increase in surface temperature may be due to high UV absorption. Moreover, an increase in particle size leads to high light-scattering ability, further improving the light-harvesting ability.

A Systematic Indoor and Outdoor Study of the Effect of Particle Size and Concentration of TiO2 in Improving Solar Absorption for Solar Still Application

Mil-101 (Cr) material is considered to be one of the most potential thermochemical energy storage materials in recent years. It has the advantages of a typical S-type water adsorption isotherm. Has low working temperature and large water adsorption amount. However, the adsorption properties of the material need to be improved under low water pressure. To improve the performance, the acidified MWCNTs were added before the hydrothermal reaction of mil-101 (Cr) materials to optimize the micropore structure. After the preparation, the new composite thermochemical energy storage materials were prepared by impregnation with a certain concentration of calcium chloride aqueous solution. The effects of multi-walled carbon nanotubes and calcium chloride on the physical and chemical properties of the materials were discussed. Through X-ray diffraction experiment, scanning electron microscope, microstructure analysis, nitrogen adsorption capacity test, water adsorption capacity test, and other means, the micro characteristics, pore structure, crystal morphology, and chemical composition of the materials were characterized, and the water adsorption performance of the materials was synthesized. The experimental results show that the addition of carbon nanotubes can improve the pore properties of the materials so that the porous organic skeleton can accommodate more calcium chloride particles. The composite materials with calcium chloride can provide excellent hydrophilicity and high water adsorption capacity. Also, the water absorption rate and adsorption-desorption cycle capacity of the material have been significantly improved. The experimental results show that when the mass fraction of calcium chloride reaches 30% when 90 mg acidified MWCNTs are added into every four chromium nitrate crystals, the hydrophilicity and adsorption capacity of the composite can reach a high equilibrium state. At the same time, the experimental yield of the material is high, the product is easy to obtain, the environmental friendliness is also reflected, so it is expected to become the adsorption heat pump thermochemical energy storage material with research potential.

Mil-101 (Cr) material is considered to be one of the most potential thermochemical energy storage materials in recent years. It has the advantages of a typical S-type water adsorption isotherm. Has low working temperature and large water adsorption amount. However, the adsorption properties of the material need to be improved under low water pressure. To improve the performance, the acidified MWCNTs were added before the hydrothermal reaction of mil-101 (Cr) materials to optimize the micropore structure. After the preparation, the new composite thermochemical energy storage materials were prepared by impregnation with a certain concentration of calcium chloride aqueous solution. The effects of multi-walled carbon nanotubes and calcium chloride on the physical and chemical properties of the materials were discussed. Through X-ray diffraction experiment, scanning electron microscope, microstructure analysis, nitrogen adsorption capacity test, water adsorption capacity test, and other means, the micro characteristics, pore structure, crystal morphology, and chemical composition of the materials were characterized, and the water adsorption performance of the materials was synthesized. The experimental results show that the addition of carbon nanotubes can improve the pore properties of the materials so that the porous organic skeleton can accommodate more calcium chloride particles. The composite materials with calcium chloride can provide excellent hydrophilicity and high water adsorption capacity. Also, the water absorption rate and adsorption-desorption cycle capacity of the material have been significantly improved. The experimental results show that when the mass fraction of calcium chloride reaches 30% when 90 mg acidified MWCNTs are added into every four chromium nitrate crystals, the hydrophilicity and adsorption capacity of the composite can reach a high equilibrium state. At the same time, the experimental yield of the material is high, the product is easy to obtain, the environmental friendliness is also reflected, so it is expected to become the adsorption heat pump thermochemical energy storage material with research potential.

Composite MWCNT/ MIL-101 (Cr)/CaCl2 as High-Capacity Water Adsorbent for Long-Term Thermal Energy Storage

Searching for inexpensive, durable, and active catalysts for the dehydrogenation of ammonia borane (AB) is an important subject in the field of hydrogen energy. In this study, we have fabricated NiCo nanorod arrays anchored on copper foam (CF) by a simple hydrothermal process. The catalytic performance of those array catalysts in AB hydrolysis was studied. It was found that NiCo-1/CF showed the highest catalytic activity with a hydrogen generation rate (HGR) of 1.03 Lhydrogen g−1 min−1, which was much higher than that for the unsupported NiCo-1 catalyst. It has been demonstrated that strong base can significantly enhance hydrogen production. After six catalytic cycles, the morphology, crystal structure, and catalytic activity were maintained, indicating that the NiCo-1/CF sample showed good reusability and high durability. Considering their low cost and high performance, the NiCo nanorod arrays anchored on CF can be a strong candidate catalyst for hydrogen generation for mobile hydrogen-oxygen fuel cells.

Searching for inexpensive, durable, and active catalysts for the dehydrogenation of ammonia borane (AB) is an important subject in the field of hydrogen energy. In this study, we have fabricated NiCo nanorod arrays anchored on copper foam (CF) by a simple hydrothermal process. The catalytic performance of those array catalysts in AB hydrolysis was studied. It was found that NiCo-1/CF showed the highest catalytic activity with a hydrogen generation rate (HGR) of 1.03 Lhydrogen g−1&nbsp;min−1, which was much higher than that for the unsupported NiCo-1 catalyst. It has been demonstrated that strong base can significantly enhance hydrogen production. After six catalytic cycles, the morphology, crystal structure, and catalytic activity were maintained, indicating that the NiCo-1/CF sample showed good reusability and high durability. Considering their low cost and high performance, the NiCo nanorod arrays anchored on CF can be a strong candidate catalyst for hydrogen generation for mobile hydrogen-oxygen fuel cells.

NiCo Nanorod Array Supported on Copper Foam as a High-Performance Catalyst for Hydrogen Production From Ammonia Borane

Copper chalcogenides have been the subject of extensive research as promising candidates in thermoelectric conversion applications due to their advantageous properties, environmental compatibility, and abundance. The low intrinsic thermal conductivity and high electric conductivity of these materials are born out of the “phonon-liquid electron-crystal” structure between the copper and chalcogens. In this review, strategies for thermoelectric property improvement of copper chalcogenides are discussed. The challenges and the future development of binary copper chalcogenides for further thermoelectric conversion applications are outlined.

Copper chalcogenides have been the subject of extensive research as promising candidates in thermoelectric conversion applications due to their advantageous properties, environmental compatibility, and abundance. The low intrinsic thermal conductivity and high electric conductivity of these materials are born out of the &#x201c;phonon-liquid electron-crystal&#x201d; structure between the copper and chalcogens. In this review, strategies for thermoelectric property improvement of copper chalcogenides are discussed. The challenges and the future development of binary copper chalcogenides for further thermoelectric conversion applications are outlined.

Exploring Thermoelectric Property Improvement for Binary Copper Chalcogenides

The 3D ternary Li2GeO3 compound, which could serve as the electrolyte material in Li+-based batteries, exhibits an unusual lattice symmetry (orthorhombic crystal), band structure, charge density distribution and density of states. The essential properties are fully explored through the first-principles method. In the delicate calculations and analyses, the main features of atom-dominated electronic energy spectrum, space-charge distribution, and atom-/orbital-projected density of states are sufficient to identify the critical multi-orbital hybridizations of the chemical bonds: 2s-(2px, 2py, 2pz) and (4s, 4px, 4py, 4pz)-(2s, 2px, 2py, 2pz), respectively, for Li-O and Ge-O. This system possesses a large indirect gap of Eg = 3.77 eV. There exist a lot of significant covalent bonds, with an obvious non-uniformity and anisotropy. In addition, spin-dependent magnetic configurations are completely absent. The theoretical framework could be developed to investigate the important features of anode and cathode materials related to lithium oxide compounds.

The 3D ternary Li2GeO3 compound, which could serve as the electrolyte material in Li+-based batteries, exhibits an unusual lattice symmetry (orthorhombic crystal), band structure, charge density distribution and density of states. The essential properties are fully explored through the first-principles method. In the delicate calculations and analyses, the main features of atom-dominated electronic energy spectrum, space-charge distribution, and atom-/orbital-projected density of states are sufficient to identify the critical multi-orbital hybridizations of the chemical bonds: 2s-(2px, 2py, 2pz) and (4s, 4px, 4py, 4pz)-(2s, 2px, 2py, 2pz), respectively, for Li-O and Ge-O. This system possesses a large indirect gap of Eg = 3.77 eV. There exist a lot of significant covalent bonds, with an obvious non-uniformity and anisotropy. In addition, spin-dependent magnetic configurations are completely absent. The theoretical framework could be developed to investigate the important features of anode and cathode materials related to lithium oxide compounds.

Geometric and Electronic Properties of Li2GeO3

The synthesis of donor (D)-acceptor (A) polymers using structurally elaborated monomers is an active research field. Some of the challenges with the use of alternating D-A polymers for photovoltaic applications are the relatively narrow absorption widths, the presence an absorption valleys in the visible region, unoptimized molecular energy levels and even lack of compatibility of the polymers with the common acceptors. The synthesis and characterization of polymers consisting of multiple chromophores (random and regular terpolymers) with complementing properties is currently gaining momentum in order to delicately optimize properties of polymers. A random terpolymer can either be of a system composed of one donor and two acceptors [(D-A1)-ran-(D-A2)] or a one acceptor and two donor segments [(D1-A)-ran-(D2-A)] incorporated in the polymer backbone. By varying the composition of the monomers in the feed of the polymerization reaction, the properties of the resulting terpolymers can be carefully optimized. Using this strategy, many materials with desired properties have been developed and power conversion efficiency (PCE) surpassing 14% in a single layer bulk heterojunction (BHJ) solar cell device have been reported. This review summarizes the most recent advances made in the development of electron donor terpolymers for organic photovoltaics (OPVs). The properties of the terpolymers are compared with their respective reference polymer.

The synthesis of donor (D)-acceptor (A) polymers using structurally elaborated monomers is an active research field. Some of the challenges with the use of alternating D-A polymers for photovoltaic applications are the relatively narrow absorption widths, the presence an absorption valleys in the visible region, unoptimized molecular energy levels and even lack of compatibility of the polymers with the common acceptors. The synthesis and characterization of polymers consisting of multiple chromophores (random and regular terpolymers) with complementing properties is currently gaining momentum in order to delicately optimize properties of polymers. A random terpolymer can either be of a system composed of one donor and two acceptors [(D-A1)-ran-(D-A2)] or a one acceptor and two donor segments [(D1-A)-ran-(D2-A)] incorporated in the polymer backbone. By varying the composition of the monomers in the feed of the polymerization reaction, the properties of the resulting terpolymers can be carefully optimized. Using this strategy, many materials with desired properties have been developed and power conversion efficiency (PCE) surpassing 14% in a single layer bulk heterojunction (BHJ) solar cell device have been reported. This review summarizes the most recent advances made in the development of electron donor terpolymers for organic photovoltaics (OPVs). The properties of the terpolymers are compared with their respective reference polymer.

Recent Advances in the Synthesis of Electron Donor Conjugated Terpolymers for Solar Cell Applications

We have developed two series of p-type conjugated polymers based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) polymeric backbone utilizing polar pendant groups, i.e., tertiary amine and pyridine, to achieve switchable solubility in water and ethanol. By balancing the ratio between polar and non-polar side-groups, we could combine green-solvent processability with the manufacturing of functional photovoltaic devices. Due to the unavailability of water/alcohol soluble acceptors, the photovoltaic performance of these new polymers was evaluated using organic solvent by incorporating PC61BM. For water/alcohol soluble partial amine-based polymers, we achieve a maximum power conversion efficiency (PCE) of ∼0.8% whereas alcohol soluble partial pyridine-based polymers show enhanced PCE of ∼1.3% with inverted device structure. We propose that the enhancement in PCE is a result of the reduction in amino-group content and the lower basicity of pyridine, both of which decrease the interaction between functionalized polymers with the anode interface material and reduce the miscibility of the donor and acceptor. Further improvement of the photovoltaic performance, in particular the open-circuit voltage (Voc), was achieved by using an anode buffer layer to mitigate the unfavorable interaction of the amino/pyridine groups with the MoO3 electrode. Our work demonstrated the possibility of substituent modification for conjugated polymers using tertiary amine and pyridine groups to achieve water/alcohol soluble and functional donor materials.

We have developed two series of p-type conjugated polymers based on poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) polymeric backbone utilizing polar pendant groups, i.e., tertiary amine and pyridine, to achieve switchable solubility in water and ethanol. By balancing the ratio between polar and non-polar side-groups, we could combine green-solvent processability with the manufacturing of functional photovoltaic devices. Due to the unavailability of water/alcohol soluble acceptors, the photovoltaic performance of these new polymers was evaluated using organic solvent by incorporating PC61BM. For water/alcohol soluble partial amine-based polymers, we achieve a maximum power conversion efficiency (PCE) of ∼0.8% whereas alcohol soluble partial pyridine-based polymers show enhanced PCE of ∼1.3% with inverted device structure. We propose that the enhancement in PCE is a result of the reduction in amino-group content and the lower basicity of pyridine, both of which decrease the interaction between functionalized polymers with the anode interface material and reduce the miscibility of the donor and acceptor. Further improvement of the photovoltaic performance, in particular the open-circuit voltage (Voc), was achieved by using an anode buffer layer to mitigate the unfavorable interaction of the amino/pyridine groups with the MoO3 electrode. Our work demonstrated the possibility of substituent modification for conjugated polymers using tertiary amine and pyridine groups to achieve water/alcohol soluble and functional donor materials.

Water/Ethanol Soluble p-Type Conjugated Polymers for the Use in Organic Photovoltaics

Lithium titanate (Li4Ti5O12, LTO) has emerged as an alternative anode material for rechargeable lithium ion (Li+) batteries with the potential for long cycle life, superior safety, better low-temperature performance, and higher power density compared to their graphite-based counterparts. LTO, being a “zero-strain” material, shows almost no volume change (<1%) during lithium ion insertion/extraction and hence offers excellent cycling stability (over 20,000 cycles). LTO anodes were popular initially on the belief that the anode electrolyte interface would be free of any solid electrolyte interphase (SEI) layer; however, this was found not to be the case. Rather, recent studies have reported different types of deposits and layer formations on the surface of LTO electrodes, and therefore this topic has received significant attention in recent years and has emerged as an important research direction. However, these anodes, being very active catalysts, are prone to undesirable reactions with electrolytes and problems such as gas evolution, and associated swelling of the battery pack may occur. Also, the electrolytes have been found to be one of the primary sources of problems, given that the electrolytes may react with both the anode and cathode, creating serious stability and safety concerns. The presence of moisture within the battery system, decomposition of the electrolyte solvents and solutes, and high catalytic activity of the anode are among the possible reasons behind the instability of LTO-based batteries. Development of an appropriate chemical composition for the electrolyte and/or modification of the electrode/electrolyte interface may overcome these issues.

Lithium titanate (Li4Ti5O12, LTO) has emerged as an alternative anode material for rechargeable lithium ion (Li+) batteries with the potential for long cycle life, superior safety, better low-temperature performance, and higher power density compared to their graphite-based counterparts. LTO, being a “zero-strain” material, shows almost no volume change (&lt;1%) during lithium ion insertion/extraction and hence offers excellent cycling stability (over 20,000 cycles). LTO anodes were popular initially on the belief that the anode electrolyte interface would be free of any solid electrolyte interphase (SEI) layer; however, this was found not to be the case. Rather, recent studies have reported different types of deposits and layer formations on the surface of LTO electrodes, and therefore this topic has received significant attention in recent years and has emerged as an important research direction. However, these anodes, being very active catalysts, are prone to undesirable reactions with electrolytes and problems such as gas evolution, and associated swelling of the battery pack may occur. Also, the electrolytes have been found to be one of the primary sources of problems, given that the electrolytes may react with both the anode and cathode, creating serious stability and safety concerns. The presence of moisture within the battery system, decomposition of the electrolyte solvents and solutes, and high catalytic activity of the anode are among the possible reasons behind the instability of LTO-based batteries. Development of an appropriate chemical composition for the electrolyte and/or modification of the electrode/electrolyte interface may overcome these issues.