## 3 Non-Optical Applications of Functional Materials with 3D Periodic Nanostructures
### Electrodes for Electrocatalytic Reactions  
In connection with the energy depletion issue, interest in water splitting technology, one of the future energy systems, is gradually increasing. Electrolysis of water is the breakdown of water into oxygen and hydrogen due to an electric current passing through the water. Since electrolysis relies on surface chemical reactions at electrodes, it is of the utmost importance to increase the surface area or active site of the electrodes. To achieve this, many attempts have been made to increase SVR by nanostructuring bulk-type electrocatalysts. In particular, 3D periodic nanostructures defined by optical 3D lithography have several advantages that can overcome the performance limitations of conventional bulk-type electrocatalysts. For example, 3D periodic nanostructures have a relatively high SVR to solid structures and provide additional paths for mass transport, thus promoting the diffusion of reactants inside the electrocatalysts and maximizing the surface chemical reactions. In addition, the spatial confinement effect resulting from the nanopore structure can reduce the size of oxygen gas bubbles, which are generated during the oxygen evolution reaction (OER), leading to improved long-term stability of the electrocatalytic reaction. In order to maximize the bubble suppression effect, precise control of the nanopore structure is desired. For these reasons, researchers are attempting to apply 3D nanostructured inorganic materials produced by a combination  
Figure 7: Non-optical applications of 3D nanostructured functional materials: (**a**) Schematic illustration of the interdigitated electrodes with lithiated MnO\({}_{2}\) (LMO) cathode and Ni-Sn anode; (**b**) Digital image of microbattery setup; (**c**) Capacity as a function of electrode digit (reprinted with permission from [53]. Copyright 2015 The National Academy of Science); (**d**) Schematic illustration of fabrication processes of a 3D-nanomesh (NM) Ni electrode (reprinted with permission from [82]. Copyright 2018 Elsevier); (**e**) Schematic illustration of the fabrication processes of a 3D Ni with 2D MoS\({}_{2}\) (reprinted with permission from [86]. Copyright 2019 Elsevier); (**f**) Schematic illustration of the expected reaction pathway of CO\({}_{2}\) reduction (reprinted with permission from [83]. Copyright 2020 The National Academy of Science); (**g**) Schematic illustration and digital image of a 3D Bi-Sb-Te thermoelectric film; (**h**) Digital and optical microscope images of a 3D Bi-Sb-Te thermoelectric film transferred on a microfabricated thermoelectric measurement platform (MTMP); (i) Enhancement of thermoelectric performance of a 3D Bi-Sb-Te film compared to an ordinary Bi-Sb-Te film (reprinted with permission from [38]. Copyright 2017 The Royal Society of Chemistry). of optical 3D lithography and electroplating to electrocatalysts for water splitting. Recently, [PERSON] et al. reported a large-scale, NiFe-decorated 3D ordered nanoporous Ni electrode for water splitting (Figure 7d) [82]. To achieve this, PnP was used to fabricate an inch-scale polymer template with 3D periodic nanostructures and the template was converted to a nickel-based material through template-assisted electroplating. The resulting 3D ordered nanoporous electrode showed excellent OER performance, with an overpotential of 261 mV at 10 mA\(\cdot\)cm\({}^{-2}\), which is a superior property compared to a conventional Ni foam. The calculated turnover frequency (TOF) of the developed electrode was 2.9 s\({}^{-1}\) at an overpotential of 300 mV, the highest among nickel-based OER electrocatalysts. Interestingly, the developed electrode effectively suppressed the generation of gas bubbles during the OER reaction due to the spatial confinement effect, greatly improving the long-term stability over 300 h without performance degradation. In a similar way, [PERSON] et al.