Functional Nanostructuring for Efficient Energy Conversion and Storage

Inevitable fossil fuel depletion has made utilization of renewable energy extremely important. The utilization of various renewable energy sources has enormous impact on social and economic life, which has directed a global trend towards sustainable development. In the US, the Department of Energy's goal for the cost of energy applications on electric grids to achieve 20% wind penetration is $100 kW h−1 by 2030.1 In Europe, the EU's renewable energy directive sets a binding target of 20% final energy consumption from renewable sources by 2020 and further increases this percentage to at least 27% by 2030.2 In response to the needs of modern society and emerging ecological concerns along with the sustainable development, energy conversion and storage are essential yet highly challenging to realize efficient utilization in the aspects from fundamental materials science to advanced device design and integration.3 So far tremendous efforts have been devoted in the fields such as solar cells, generators, fuel cells, batteries, supercapacitors, and (photo)electrochemical water splitting. It is unquestionable that interdisciplinary and innovative technologies of materials science, chemistry and physics lie at the heart of the advances that have been made in energy conversion and storage. In particular, nanostructured materials differ from bulk and microsized materials not only in the scale of their own characteristic dimensions, but also in the fact that they offer unique physical or chemical properties or combinations of properties and thus offer new possibilities in a wide range of energy-related applications. Therefore, nanostructured materials are becoming increasingly important in the research community of energy conversion and storage and have attracted great attention in recent years.4-6 It is safe to say that nanostructuring may hold the key to the new generations of such devices because it leads to unusual mechanical, electrical and optical properties endowed by confining or patterning the dimensions of materials in the nanometer-scale. In some cases, the effects are simple consequences of size reduction and thus advanced strategies are developed to effectively downsize the materials. In more cases, the effects involve internally nanostructured materials with particular functions and emerging concepts are proposed to bridge the functional nanostructures and the underlying mechanisms. Combining advanced strategies and emerging concepts with bulk and surface properties to the overall behavior, one can envision more breakthroughs and staggering developments that are triggered by functional nanostructuring for energy conversion and storage. Therefore, we are delighted to present this special issue of Advanced Energy Materials “Functional Nanostructuring for Efficient Energy Conversion and Storage”. This issue is composed of invited contributions in the Third International Conference on the Challenges and Perspectives of Functional Nanostructures (CPFN) that took place in June 2016 in Ilmenau, Germany. The aim of the conference is to discuss functional nanostructuring and surface nano-patterning as well as their micro-/nano-integration in order to solve fundamental crosscutting research issues that are especially related to energy and electronic devices. During the conference, an important focus was placed on the advancement of efficient energy conversion and storage, to which this special issue is dedicated. In total eight Reviews, one Progress Report, one Research News, and two Full Papers are included in the special issue, which cover different prominent and intriguing concepts and strategies of functional nanostructuring designed for achieving efficient energy conversion and storage. This special issue shall be a timely opportunity to highlight exciting recent progresses in the related research areas. Efficient energy conversion and storage involves complex objects such as electrified solid/liquid or solid/solid interfaces that nanostructures are coping with. Properties of these largely determine various characteristics that functional nanostructuring needs to achieve, and these characteristics are mostly crucial for the actual processes happening during reactions. As a result, deep understanding of relations between these complex objects and functional nanostructures is of paramount importance to realize high efficiency for energy conversion and storage devices. This special issue highlights emerging concepts to understand the nature of complex energy systems and advanced strategies to obtain various characteristics through functional structuring. Combination of innovative concepts and advanced strategies can provide us comprehensive understanding of these relations thus insights into future development of the devices. Two emerging concepts are presented to illustrate the breadth of high quality works and the important roles that deep exploration of the properties of nanomaterials plays. Firstly, triboelectric nanogenerators (TENGs) have been recently invented as a highly efficient, cost-effective and easy scalable energy-harvesting technology for converting ambient mechanical energy from our living environment (e.g., wind, sound, flowing water, human motion) into electricity and performing electrochemistry without using an external power.7 The updated progress in TENGs for both high-performance energy conversion and self-powered electrochemical systems in various applications are reviewed by Z. L. Wang and his colleagues. Secondly, two dimensional (2D) inorganic materials have bred a series of energy applications because of their unique physicochemical properties.8, 9 The development and state-of-the-art in the research on 2D nanosheets derived from layered metal oxides and layered metal hydroxides are reviewed by J.E. ten Elshof et al. with focusing on the synthesis and controlled assembly of 2D nanosheets in energy applications. Meanwhile, graphene analogues have shown promise in supercapacitors and batteries, and an overview of current research and important advances on this subject is provided by P. Samorì et al. In the full article by Y. Xie et al., pore-rich WO3 ultrathin nanosheets with nearly fully exposed highly active crystal facets are proposed to demonstrate the merits of structural engineering of 2D materials towards high-efficient energy conversion devices. Furthermore, in order to fill the gap between the unsatisfied physicochemical property of virgin 2D inorganic materials and increasing needs of modern energy applications, vacancy can be utilized to tremendously manipulate the intrinsic properties of the 2D materials.10 Regulation of their electron and phonon behavior by vacancy engineering is discussed by Y. Xie and her colleagues, which highlights the design philosophy of “less is more”. As important as emerging concepts for understanding the nature of the complex energy systems, advanced strategies for functional nanostructuring provide effective ways to fabricate and integrate nanomaterials. With regard to fabrication, atomic layer deposition (ALD) is a promising strategy in surface and interfacial engineering for fabricating nanomaterials, by taking the advantages of precise thickness and compositional control as well as conformal coating of complex geometries.11 The application of ALD for energy-related devices is reviewed by Y. Lei and his colleagues. Besides deposition processing, crystallization control has played critical role on modulating electrochemical properties of nanomaterials.12, 13 In this regard, new synthesis routes and recent progresses of amorphous vanadium oxide materials for metal batteries are summarized by S. Passerini et al. The critical role that the orderliness of the atomic arrangement plays in electrochemical processes is discussed in the full paper from Y. Lei et al., taking one of the most intensively studied Na-ion battery electrode materials, anatase TiO2 as an example. By combining experimental synthesis, in-situ characterization and theoretical analyzation, latest advances and new insights in NiFe-based (oxy)hydroxide oxygen evolution reaction (OER) catalyst are highlighted by P. Strasser et al. With regard to integration, a unique rolled-up nanotechnology employing strain engineering has been developed by O. G. Schmidt and his co-workers to self-assemble nanomembranes into three-dimensional (3D) structures.14 In their progress report, they present a comprehensive summary on the fabrication and application of diverse rolled-up structures with the focus on employing rolled-up nanotechnology to design rational electrodes for Li-ion and Li-O2 batteries. Integration of nanomaterials into nanowire and nanorod array structures offers desirable characteristics in structure, function, and applicability. Current status of research on nanowire and nanorod array structures for photocatalytic energy conversion and utilization is reviewed by P. X Gao et al. Last but not least, microbial electrochemical systems are outlined in the research news of A. Schober and his colleagues to demonstrate the potential influence of nanotechnological approaches on designing such systems and shed light on future perspectives of using nanobiosystems as scalable systems. We are highly motivated by the rapid progress in functional nanostructuring towards efficient energy conversion and storage, and the goal of this special issue is to provide a detailed survey of the state-of-the-art of concepts and strategies in this exciting area. Considering the limited number of papers included in this special issue, we believe that some other topics that are equally important in the energy-related area remain to be treated, and we will continue making every effort to put forth more comprehensive summaries to readers in the future. As the guest editor of this special issue, I would like to express my deepest gratitude to all the authors for their high-quality contributions. We are lucky that we are now on the fast lane for advancing efficient utilization of renewable energy resources, which, in return, gives us unexpected challenges to fulfill but wide open space to explore. By highlighting the most recent encouraging progresses achieved in functional nanostructuring towards efficient energy conversion and storage devices, this special issue could provide a perspective as well as a valuable insight into the future research topics and the fundamental issues for the research community working in this exciting field. This work is financially supported by European Research Council Grant (ThreeDSurface, 240144), European Research Council (Proof of Concept) Grant (HiNaPc, 737616), Federal Ministry of Education and Research in Germany (BMBF: ZIK-3DNano-Device: 03Z1MN11), and German Research Foundation (DFG: LE 2249_4-1). Yong Lei received his Ph.D. degree from the Chinese Academy of Sciences in 2001. After two years of postdoc research at Singapore-MIT Alliance, he worked as an Alexander von Humboldt Fellow and a research scientist at the Karlsruhe Institute of Technology (KIT) in 2003–2006. Later he worked at the University of Muenster (WWU) as a group leader (2006–2009) and a W1 professor (2009–2011). In 2011, he joined the Ilmenau University of Technology as a Chair Professor. His current research interests focus on template-based nanostructuring, energy conversion and storage devices and optoelectronic applications of functional nanostructures and surface nano-patterns.