The importance of crystallographic pathways was nicely illustrated in a detailed study of the electrochemical behaviour of MnO2 allotropic phases with different characteristic geometries (Fig. 2a): one-dimensional (1D) channels, two-dimensional (2D) layers and three-dimensional (3D) interconnecting channels12. Since the chemical composition was fixed, the effect of structural and chemical parameters was evaluated. It was shown that an increase in channel size and connectivity resulted in improved electrochemical performance (Fig. 2). SSA had a marginal effect on the resulting capacitance, presumably because charge storage occurred from redox processes and not from the EDL. Interestingly, strong correlation was found between capacitance and ionic conductivity of different MnO2 phases. Ionic conductivity is directly influenced by the crystal structure of the material and, when aqueous electrolytes are utilized, the presence of structural water1314. Conclusions from this study can be extended beyond MnO2, highlighting the crucial importance of material architecture. Due to the reasons noted above, it is possible to identify structural features, such as a narrow distribution of straight pores, a good match between the pore size and ion size, interconnectivity of pores and accessibility of redox-active sites, which are required for maximizing performance. Materials possessing these features offer considerable promise for energy storage applications: (i) 2D materials that contain transition metals (such as layered transition metal oxides12, carbides15 and dichalcogenides16) and (ii) materials with 3D interconnected channels (such as T-Nb2O5 (ref. 17 or MnO2 spinel12). Combinations of different mechanisms described above in EES devices lead to improved rate performance and the ability to store and deliver larger amounts of energy over extended periods of time, as described in the following section.