Pseudo 2-dimensional nanostructures of metal oxides for high-performance supercapacitors

Recently, high performance supercapacitors based on hollow nanostructures have been reported. The enhanced behaviours are directly attributed to the higher specific surface area of hollow particles. It is shown here that this is an oversimplified explanation. If only the surface area was driving the enhancement then other solid morphologies, with a higher specific area, should show better performance. Careful modelling and simulation studies show that hollow structures can actually be viewed as pseudo 2-dimensional (2-D) materials. This leads to performance similar to that reported in 2-dimensional materials ranging from metal oxides to MXenes. Therefore, the underlying reasons, inducing the performance enhancement, are explained for the first time. The theoretically predicted behaviour is proven experimentally by using CeO2 hollow and solid nanostructures as the test samples. The result also establishes the importance of using hollow structures of rare-earth oxides, if they are to be used in supercapacitors. Finally, it is shown that the future direction should involve the combination of a pseudo 2-D structure with a real 2-D-carbon based electrode. The use of 2-D gC(3)N(4), with a lower specific surface area than activated carbon but much higher specific capacitance, reaffirms the inferences that the role of the specific surface area is mostly over-estimated in supercapacitors.