As a concluding remark, we separately highlight some of the research directions that in our opinion are essential for the further advancement of EES but are still underdeveloped. Understanding the influence of surface chemistry on the electrochemical properties of materials. On nanostructuring for improved material and device performance, the influence of the surface increases markedly. Can we control capacity levels through the design of surface chemistry?New electrolytes. Relatively few electrolytes are used in conventional Li-ion batteries and supercapacitors, yet once we have created open and accessible electrode structures, electrolytes that contain bulkier ions, such as ionic liquids, can be used effectively. At the same time, the introduction of new energy-storing materials and their designed surface chemistries may require the development of new, compatible electrolytes that operate over a wide voltage window. Efforts should also be directed towards developing new multi-electron redox electrolytes.Self-discharge limits long-term energy storage in capacitive electrodes and leads to irreversible energy losses90. However, there is relatively little fundamental understanding of the self-discharge mechanisms and how it can be minimized in pseudocapacitive and hybrid systems, in devices containing redox electrolytes and in open 3D and 2D architectures. In particular, the use of redox electrolytes and open architectures may lead to faster self-discharge and novel approaches to mitigating this effect are necessary. Effects of electrode thickness, dimensionality and chemistry among other factors are likely to be significant and need further study.Mechanisms of performance degradation are poorly understood, especially for new material systems. Understanding how material properties degrade will help to expand the lifetime of EES devices.