2017 Vol. 2, No. 3

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Editorial
Energy storage and conversion: Driving human development
2017, 2(3) doi: 10.1016/j.gee.2017.07.001
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Short review
Characterization of Li-rich layered oxides by using transmission electron microscope
Hu Zhao, Bao Qiu, Haocheng Guo, Kai Jia, Zhaoping Liu, Yonggao Xia
2017, 2(3): 174-185. doi: 10.1016/j.gee.2017.05.005
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Lithium-rich layered oxides (LrLOs) deliver extremely high specific capacities and are considered to be promising candidates for electric vehicle and smart grid applications. However, the application of LrLOs needs further understanding of the structural complexity and dynamic evolution of monoclinic and rhombohedral phases, in order to overcome the issues including voltage decay, poor rate capability, initial irreversible capacity loss and etc. The development of aberration correction for the transmission electron microscope and concurrent progress in electron spectroscopy, have fueled rapid progress in the understanding of the mechanism of such issues. New techniques based on the transmission electron microscope are first surveyed, and the applications of these techniques for the study of the structure, migration of transition metal, and the activation of oxygen of LrLOs are then explored in detail, with a particular focus on the mechanism of voltage decay.
Review article
Understanding oxygen electrochemistry in aprotic LiO2 batteries
Liang Wang, Yantao Zhang, Zhenjie Liu, Limin Guo, Zhangquan Peng
2017, 2(3): 186-203. doi: 10.1016/j.gee.2017.06.004
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In the past decade, the aprotic lithium–oxygen (LiO2) battery has generated a great deal of interest because theoretically it can store more energy than today's lithium-ion batteries. Although considerable research efforts have been devoted to the R&D of this potentially disruptive technology, many scientific and engineering obstacles still remain to be addressed before a practical device could be realized. In this review, we summarize recent advances in the fundamental understanding of the O2 electrochemistry in LiO2 batteries, including the O2 reduction to Li2O2 on discharge and the reverse Li2O2 oxidation on recharge and factors that exert strong influences on the redox of O2/Li2O2. In addition, challenges and perspectives are also provided for the future study of LiO2 batteries.
A review on photo-thermal catalytic conversion of carbon dioxide
Ee Teng Kho, Tze Hao Tan, Emma Lovell, Roong Jien Wong, Jason Scott, Rose Amal
2017, 2(3): 204-217. doi: 10.1016/j.gee.2017.06.003
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The conversion of carbon dioxide into value-added products is of great industrial and environmental interest. However, as carbon dioxide is relatively stable, the input energy required for this conversion is a significant limiting factor in the system's performance. By utilising energy from the sun, through a range of key routes, this limitation can be overcome. In this review, we present a comprehensive and critical overview of the potential routes to harvest the sun's energy, primarily through solar-thermal technologies and plasmonic resonance effects. Focusing on the localised heating approach, this review shortlists and compares viable catalysts for the photo-thermal catalytic conversion of carbon dioxide. Further, the pathways and potential products of different carbon dioxide conversion routes are outlined with the reverse water gas shift, methanation, and methanol synthesis being of key interest. Finally, the challenges in implementing such systems and the outlook to the future are detailed.
Synthesis and applications of MOF-derived porous nanostructures
Min Hui Yap, Kam Loon Fow, George Zheng Chen
2017, 2(3): 218-245. doi: 10.1016/j.gee.2017.05.003
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Metal organic frameworks (MOFs) represent a class of porous material which is formed by strong bonds between metal ions and organic linkers. By careful selection of constituents, MOFs can exhibit very high surface area, large pore volume, and excellent chemical stability. Research on synthesis, structures and properties of various MOFs has shown that they are promising materials for many applications, such as energy storage, gas storage, heterogeneous catalysis and sensing. Apart from direct use, MOFs have also been used as support substrates for nanomaterials or as sacrificial templates/precursors for preparation of various functional nanostructures. In this review, we aim to present the most recent development of MOFs as precursors for the preparation of various nanostructures and their potential applications in energy-related devices and processes. Specifically, this present survey intends to push the boundaries and covers the literatures from the year 2013 to early 2017, on supercapacitors, lithium ion batteries, electrocatalysts, photocatalyst, gas sensing, water treatment, solar cells, and carbon dioxide capture. Finally, an outlook in terms of future challenges and potential prospects towards industrial applications are also discussed.
A comprehensive review on recent progress in aluminum–air batteries
Yisi Liu, Qian Sun, Wenzhang Li, Keegan R. Adair, Jie Li, Xueliang Sun
2017, 2(3): 246-277. doi: 10.1016/j.gee.2017.06.006
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The aluminum–air battery is considered to be an attractive candidate as a power source for electric vehicles (EVs) because of its high theoretical energy density (8100 Wh kg−1), which is significantly greater than that of the state-of-the-art lithium-ion batteries (LIBs). However, some technical and scientific problems preventing the large-scale development of Al–air batteries have not yet to be resolved. In this review, we present the fundamentals, challenges and the recent advances in Al–air battery technology from aluminum anode, air cathode and electrocatalysts to electrolytes and inhibitors. Firstly, the alloying of aluminum with transition metal elements is reviewed and shown to reduce the self-corrosion of Al and improve battery performance. Additionally for the cathode, extensive studies of electrocatalytic materials for oxygen reduction/evolution including Pt and Pt alloys, nonprecious metal catalysts, and carbonaceous materials at the air cathode are highlighted. Moreover, for the electrolyte, the application of aqueous and nonaqueous electrolytes in Al–air batteries are discussed. Meanwhile, the addition of inhibitors to the electrolyte to enhance electrochemical performance is also explored. Finally, the challenges and future research directions are proposed for the further development of Al–air batteries.
Research paper
Ultra-light and flexible pencil-trace anode for high performance potassium-ion and lithium-ion batteries
Zhixin Tai, Yajie Liu, Qing Zhang, Tengfei Zhou, Zaiping Guo, Hua Kun Liu, Shi Xue Dou
2017, 2(3): 278-284. doi: 10.1016/j.gee.2017.04.002
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Engineering design of battery configurations and new battery system development are alternative approaches to achieve high performance batteries. A novel flexible and ultra-light graphite anode is fabricated by simple friction drawing on filter paper with a commercial 8B pencil. Compared with the traditional anode using copper foil as current collector, this innovative current-collector-free design presents capacity improvement of over 200% by reducing the inert weight of the electrode. The as-prepared pencil-trace electrode exhibits excellent rate performance in potassium-ion batteries (KIBs), significantly better than in lithium-ion batteries (LIBs), with capacity retention of 66% for the KIBvs. 28% for the LIB from 0.1 to 0.5 A g−1. It also shows a high reversible capacity of ∼230 mAh g−1 at 0.2 A g−1, 75% capacity retention over 350 cycles at 0.4 A g−1and the highest rate performance (based on the total electrode weight) among graphite electrodes for K+ storage reported so far.
Metal porphyrin intercalated reduced graphene oxide nanocomposite utilized for electrocatalytic oxygen reduction
Mingyan Wang, Qing Wang, Wei Zhu, Ying Yang, Huixian Zhou, Fan Zhang, Lihua Zhou, Joselito M. Razal, Gordon G. Wallace, Jun Chen
2017, 2(3): 285-293. doi: 10.1016/j.gee.2017.06.001
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In this paper, we report a simple and facile self-assembly method to successfully fabricate cationic metal porphyrin –MtTMPyP (Mt= Cobalt (II), Manganese (III), or Iron (III); TMPyP = 5, 10, 15, 20-tetrakis (N-methylpyridinium-4-yl) porphyrin) intercalated into the layer of graphene oxide (GO) by the cooperative effects of electrostatic and π–π stacking interaction between positively charged metal porphyrin and negatively charged GO sheets. Followed by reduction with hydrazine vapor, a series of novel 2D MtTMPyP/rGOn were fabricated. The as-prepared 2D hybrids were fully characterized and tested as non-noble metal catalysts for oxygen reduction reaction (ORR) in an alkaline medium. The MtTMPyP/rGOn hybrids, especially CoTMPyP/rGO5, demonstrated an improved electrocatalytic activity for ORR and a number of exchanged electrons close to 4-electron reaction, increased stability and excellent tolerance to methanol, showing a potential alternative catalyst for ORR in fuel cells and air batteries.
Proton dynamics in phosphotungstic acid impregnated mesoporous silica proton exchange membrane materials
Krystina Lamb, Richard A. Mole, Dehong Yu, Roland de Marco, John R. Bartlett, Sarah Windsor, San Ping Jiang, Jin Zhang, Vanessa K. Peterson
2017, 2(3): 294-301. doi: 10.1016/j.gee.2017.06.007
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Phosphotungstic acid is an excellent proton conductor that can be incorporated into porous supports, and nanocomposite proton exchange membrane materials made from mesoporous silica impregnated with phosphotungstic acid have been suggested for use in fuels cells operating > 100 °C. In this work, quasielastic neutron scattering was used to study proton self-diffusion in mesoporous disordered and P6mm symmetry silica impregnated with two concentrations of phosphotungstic acid. Overall, the silica structure had a significantly greater effect on proton conduction and diffusion than phosphotungstic acid concentration, with higher proton conduction occurring for the P6mm symmetry silica samples. Quasielastic neutron scattering revealed two populations of protons diffusing through each sample, and that proton conduction is limited by the slower of these populations, which diffuse via a jump-diffusion mechanism. Whilst the fundamental jump-diffusion mechanism by which these slower protons moved was found to be similar for both silica supports and phosphotungstic acid concentrations, the faster diffusion occurring in P6mm structured silica arises from a lower residence time of protons moving between sites in the jump-diffusion model, suggesting a lower energy barrier.
Carbon nitride transparent counter electrode prepared by magnetron sputtering for a dye-sensitized solar cell
Chaoyu Wu, Guoran Li, Xueqin Cao, Bao Lei, Xueping Gao
2017, 2(3): 302-309. doi: 10.1016/j.gee.2017.06.002
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Carbon nitride (CNx) films supported on fluorine-doped tin oxide (FTO) glass are prepared by radio frequency magnetron sputtering, in which the film thicknesses are 90–100 nm, and the element components in the CNx films are in the range of x = 0.15–0.25. The as-prepared CNx is for the first time used as counter electrode for dye-sensitized solar cells (DSSCs), and show a preparation-temperature dependent electrochemical performance. X-ray photoelectron spectroscopy (XPS) demonstrates that there is a higher proportion of sp2 CC and sp3 CN hybridized bonds in CNx-500 (the sample treated at 500 °C) than in CNx-RT (the sample without a heat treatment). It is proposed that the sp2 CC and sp3 C–N hybridized bonds in the CNx films are helpful for improving the electrocatalytic activities in DSSCs. Meanwhile, Raman spectra also prove that CNx-500 has a relatively high graphitization level that means an increasing electrical conductivity. This further explains why the sample after the heat treatment has a higher electrochemical performance in DSSCs. In addition, the as-prepared CNx counter electrodes have a good light transmittance in the visible light region. The results are meaningful for developing low-cost metal-free transparent counter electrodes for DSSCs.
A green route to synthesize low-cost and high-performance hard carbon as promising sodium-ion battery anodes from sorghum stalk waste
Xiaoming Zhu, Xiaoyu Jiang, Xiaoling Liu, Lifen Xiao, Yuliang Cao
2017, 2(3): 310-315. doi: 10.1016/j.gee.2017.05.004
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Sodium-ion batteries (SIBs) have been considered to be potential candidates for next-generation low-cost energy storage systems due to the low-cost and abundance of Na resources. However, it is a big challenge to find suitable anode materials with low-cost and good performance for the application of SIBs. Hard carbon could be a promising anode material due to high capacity and expectable low-cost if originating from biomass. Herein, we report a hard carbon material derived from abundant and abandoned biomass of sorghum stalk through a simple carbonization method. The effects of carbonization temperature on microstructure and electrochemical performance are investigated. The hard carbon carbonized at 1300 °C delivers the best rate capability (172 mAh g −1 at 200 mA g−1) and good cycling performance (245 mAh g−1 after 50 cycles at 20 mA g−1, 96% capacity retention). This contribution provides a green route for transforming sorghum stalk waste into “treasure” of promising low-cost anode material for SIBs.
3D hollow sphere CO3O4/MnO2-CNTs: Its high-performance bi-functional cathode catalysis and application in rechargeable zinc-air battery
Xuemei Li, Nengneng Xu, Haoran Li, Min Wang, Lei Zhang, Jinli Qiao
2017, 2(3): 316-328. doi: 10.1016/j.gee.2017.02.004
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There has been a continuous need for high active, excellently durable and low-cost electrocatalysts for rechargeable zinc-air batteries. Among many low-cost metal based candidates, transition metal oxides with the CNTs composite have gained increasing attention. In this paper, the 3-D hollow sphere MnO2 nanotube-supported Co3O4 nanoparticles and its carbon nanotubes hybrid material (Co3O4/MnO2-CNTs) have been synthesized via a simple co-precipitation method combined with post-heat treatment. The morphology and composition of the catalysts are thoroughly analyzed through SEM, TEM, TEM-mapping, XRD, EDX and XPS. In comparison with the commercial 20% Pt/C, Co3O4/MnO2, bare MnO2 nanotubes and CNTs, the hybrid Co3O4/MnO2-CNTs-350 exhibits perfect bi-functional catalytic activity toward oxygen reduction reaction and oxygen evolution reaction under alkaline condition (0.1 M KOH). Therefore, high cell performances are achieved which result in an appropriate open circuit voltage (∼1.47 V), a high discharge peak power density (340 mW cm−2) and a large specific capacity (775 mAh g−1 at 10 mA cm−2) for the primary Zn-air battery, a small charge–discharge voltage gap and a high cycle-life (504 cycles at 10 mA cm−2 with 10 min per cycle) for the rechargeable Zn-air battery. In particular, the simple synthesis method is suitable for a large-scale production of this bifunctional material due to a green, cost effective and readily available process.

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