Editorial: Carbon- and Inorganic-Based Nanostructures for Energy Applications

Department of Chemistry, NIS (Nanostructured Interfaces and Surfaces) Interdepartmental Centre and INSTM Centro di Riferimento, University of Turin, Turin, Italy, Photonics and Energy Research Laboratory, Department of Chemistry, The University of Texas Rio Grande Valley, Edinburg, TX, United States, Department of Chemistry, Illinois Institute of Technology, Chicago, IL, United States, Department of Physics, The University of Texas at Arlington, Arlington, TX, United States

The transition from fossil fuels to sustainable energy sources needs more efficient materials and improved technologies. Traditional materials (either pristine or combined to form composites) are restricted by their low efficiency, low performance, short durability, environmental issues, high production costs, and narrow spectrum of applications. In recent years, the research in the field of materials for energy applications has been very active as documented by the number of scientific contributions that is trending with exponential growth ( Figure 1A).
Furthermore, from a subject area standpoint, the field of "energy materials" is very broad and its relevant research is a multidisciplinary and multifaceted activity. For example, the subject area can range from the materials for nuclear reactors harnessing nuclear energy to the materials for chemical energy storage. Even, the safety materials for energy technologies can also be considered as "energy materials." Consequently, an issue on "energy materials" may include basic and fundamental scientific studies to more applicative contributions, which are comprised in a wide variety of applications ( Figure 1B).
For this reason, it is quite difficult to provide a complete overview over the materials for energy. From the operational point of view, materials for energy conversion, transport and storage are typically included. Topics are manifold, multifaceted, and sometimes very diverse. For example, the field of the electrical energy storage is very far from fuel storage. In any case, a very wide and transversal viewpoint in a specific field can make available a remarkable innovation. As a rule, a look must always be directed towards new materials, fabricated experimentally (Uddin et al., 2014), or predicted theoretically (Sarker et al., 2013) and their properties (Zhang et al., 2019). For example, very recently the concept that uses the earth as a heat source and the night sky as a heat sink, known as thermoradiative photovoltaics, has been illustrated (Raman et al., 2019;Deppe and Munday, 2020). Materials for such devices will need a strong absorption and emission in the thermal radiation range, enough transparency to permit optical access to the night sky. Such materials have the potential to revolutionize the photovoltaics technology. Another look must always be oriented towards new characterization techniques and innovative investigation methods. For example, in the specific field of lithium (Li)-ion batteries, the machine learning assisted statistical analysis together with experiment-informed mathematical modelling have been shown very recently by Jiang et al. (2020). The authors have correlated the degree of particle detachment cathodes with the charging rate and particle sizes. Alternatively, X-ray nano-computed tomography dual-scan superimposition technique has been adopted by (Lu et al., 2020) to shed light on microstructural heterogeneities. The method has the potential to determine how the performance of the Li-ion batteries is affected under high rate conditions. The present article collection, composed of six research articles and three reviews, showcases some of the latest achievements and future perspectives in the field of the carbon-based and inorganic materials that are being designed to meet some of our energy challenges. In the present article collection, the attention to the environment is a common perspective, due to preparation methods adopted from natural resources, polymers and the attempt to minimize (or replace) metals for a more sustainable development of technologies. Cesano et al. reviewed the topic of metal-free conductors based on macrosized and nanoscale carbons (i.e., carbon fibers, carbon nanotubes, graphene) from the viewpoint of the electrical and thermal conductivity for electronic and electrical wiring applications. Specifically, CNTs and graphene can be assembled into macroscopic fibers, yarns and ropes to be used as conductors (Akia et al., 2017;Jang et al., 2020). From the perspective of replacing metals, which are present in nature with limited amounts, the role played by the chemistry in helping to exceed the electrical conductivity of metals by means of the molecular-level control and doping, is emphasized. The contribution helps to elucidate most recent results in the field, and envisages new directions and potential applications. Jangir et al. designed and fabricated a bio-hybrid electrode, which integrates Li, sulfur, and molybdenum in a nitrogen-doped reduced graphene oxide (NDRGO) matrix of biological origin. The obtained composite material works as an electrode, based on concepts of "embedded redox couples" and "induced electron transfer," behaving as a supercapacitor. Wu et al. reviewed MnO 2 /carbon composites for supercapacitors from the viewpoints of the synthesis and of the electrochemical performance. Along with this broad theme, MnO 2 has emerged as one of the most promising electrode materials for its specific capacitance, wide potential range, high electrochemical activity, and environmental friendliness. In addition, due to their low electrical resistances, significant thermal stabilities, large specific surface areas, and porosities, carbons are ideal materials to be compounded with MnO 2 . The authors show the latest findings on MnO 2 /carbon supercapacitor electrodes, focusing on the fabrication strategies and the electrochemical performance influencing factors, with an outlook on the possible development directions in future for designing highperformance materials. Kulandaivalu et al. reviewed the subject of the layered double hydroxide (LDH)/carbon nanocomposites containing Ni 2+ and Co 2+/3+ for supercapacitor applications. The combination of Ni-Co LDHs with carbon-based materials gives a remarkable improvement in the specific energy, specific capacitance, and specific power performance of supercapacitors. The authors explore important factors influencing the synthesis of LDHs and the correlation among morphology, structure, and electrochemical performances of the Ni-Co LDHs. Improvements needed to increase the performance of these new supercapacitors are also provided. Kumar et al. synthesized hexagonal copper phosphide (Cu 3 P) platelets by chemical vapor deposition technique and incorporated it in the highly conducting 3D graphene scaffold, leading to the formation of Cu 3 P/graphene hybrid to be used as supercapacitors with high Coulombic efficiency. The fabricated asymmetric supercapacitor using Cu 3 P/graphene hybrid on graphite as cathode and activated carbon on graphite as anode showed high specific capacity, energy density, remarkable power density and an excellent cycle life. Li et al. fabricated spinel Li manganese oxide (LiMn 2 O 4 ) in Liion battery. The authors observed improved stability even at elevated temperature with the addition of 1, 3-propane sultone working as an electrolyte additive compared to the cell without additive. On the other hand, Gouda et al. fabricated membrane electrode assembly in the proton exchange membrane fuel cells using graphene materials obtained from the thermal dissociation of polyethylene terephthalate, working as catalytic electrodes, and a ternary polymer blend (i.e., polyvinyl alcohol, polyethylene oxide, and polyvinyl pyrrolidone). The authors then investigated the effect of gas diffusion layers and the number of catalyst layers in three types of membrane electrode assemblies. Alternatively, semiconductor nanowires have the potential to reduce costs and  Overall, the papers published in this issue cover materials for energy. The subject is manifold, multifaceted, and sometimes very different, but we can easily conclude that the theme will have significant developments in the coming years. We truly hope that contributions published within this article collection will contribute to help to increase the value of research articles in the field of energy materials, providing inspiration for new relevant publications.

AUTHOR CONTRIBUTIONS
All authors have made a substantial contribution to the work and they approved it for publication.

FUNDING
FC acknowledges support from MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca), INSTM Consorzio and NIS (Nanostructured Interfaces and Surfaces) Inter-Departmental Centre of University of Turin. YM would like to thank the support from the Illinois Institute of Technology start-up funds. MNH was supported by the National Science Foundation Award No. 1609811. MU acknowledges the generous support from US Office of Naval Research.