The field of hybrid materials has grown so wildly in the last 30 years that writing a comprehensive review has turned into an impossible mission. Yet, the need for a general view of the field remains, and it would be certainly useful to draw a scientific and technological map connecting the dots of the very different subfields of hybrid materials, a map which could relate the essential common characteristics of these fascinating materials while providing an overview of the very different combinations, synthetic approaches, and final applications formulated in this field, which has become a whole world. That is why we decided to write this metareview, that is, a review of reviews that could provide an eagle's eye view of a complex and varied landscape of materials which nevertheless share a common driving force: the power of hybridization.

Energy storage is experiencing a renaissance as a result of the growing number of vital applications such as internet of things, smart grids, electric vehicles, renewable energy storage, etc. In particular, stationary energy storage must be urgently deployed at a large-scale to support full deployment of renewables and a sustainable grid. Electrochemical energy storage systems (EESS) will be key in this pursuit. Yet, present mature technologies are all sub-optimal. A myriad of new battery chemistries are being developed in research labs, each with specific strengths and drawbacks. EESS technologies such as lithium-ion batteries, lithium–sulfur, metal–air and other post-lithium technologies, but also supercapacitors, hybrid devices and redox flow batteries (RFBs), could benefit from collaborative development between top-down systems engineering and bottom-up material science. Lithium and post-lithium systems have been extensively researched and reviewed. This perspective article examines the energy storage landscape that goes from state-of-the-art flow cells to novel flowing and stationary technologies. While there is a wide scope for improvement of first generation RFBs, a wealth of novel concepts such as ambipolar electrolytes or shuttle cells to semisolid electrodes are also emerging. This could boost the energy density of these flowing systems. In our analysis we also propose other new concepts which could eventually lead to non-flowing large-scale electrochemical energy storage.

Here, we describe selected work on the development and study of nanofluids based on graphene and reduced graphene oxide both in aqueous and organic electrolytes. A thorough study of thermal properties of graphene in amide organic solvents (N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone) showed a substantial increase of thermal conductivity and specific heat upon graphene integration in those solvents. In addition to these thermal studies, our group has also pioneered a distinct line of work on electroactive nanofluids for energy storage. In this case, reduced graphene oxide (rGO) nanofluids in aqueous electrolytes were studied and characterized by cyclic voltammetry and charge-discharge cycles (i.e., in new flow cells). In addition, hybrid configurations (both hybrid nanofluid materials and hybrid cells combining faradaic and capacitive activities) were studied and are summarized here.

The uses of quinones in Redox Flow Batteries (RFBs) has been mainly circumscribed to aqueous solutions (of derivatives with polar groups) despite a larger solubility and wider electrochemical window provided by organic media. The redox mechanism of quinones in protic media is simpler and better known than in aprotic media, where radical species are involved. This paper reports the behaviour of methyl-p-benzoquinone (MBQ) under electrochemical reduction conditions in a LiClO[sbnd]CHCN electrolyte and various working electrodes. We detected the reversible generation of a bright green coating on the working electrode and the subsequent formation of a polymer (the nature of which depends on the presence or absence of oxygen). These coatings prevent the regular redox process of methyl-p-benzoquinone from taking place on the surface of the electrode and is generated regardless of the electrode material used or the presence of O in solution. In addition to MBQ, the green passivating layer was also found for less sterically hindered quinones such as p-benzoquinone or 1,4-naphthoquinone, but not for anthraquinone. We have also shown the central role of Li in the formation of this green layer. This work provides important guidelines for the final use of quinones in RFBs with organic electrolytes.Partial funding from Miisterio de Economía y Competitividad through Fondo Europeo de Desarrollo Regional (Grant MAT2015-68394-R, MINECO/FEDER) is acknowledged. ICN2 acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295. Finally, the award to DPD of a Marie-Curie Fellowship through Beatriu de Pinos Program (BP-DGR-2013) from the Catalan system of science and technology, Spain, is gratefully acknowledged.Peer Reviewe