**3.2 Nanoporous NiO**

In 2013, Liang et al. reported that a three-dimensional (3D) nanoporous NiO film was fabricated via a two-step process using an electrochemical route [28]. The dealloying process included electrodeposition of the Ni/Zn alloy film and electrochemical dealloying using a direct-current power source. The NiO film had an irregular 3D interconnected nanosheet structure with open channels. The specific capacitance of the NiO reached 1670 F g<sup>−</sup><sup>1</sup> at a discharge current density of 1 A g<sup>−</sup><sup>1</sup> for the supercapacitor. In addition, the NiO exhibited high performance during a long-term cycling. The maximum specific energy and specific power at the 1.1 V potential window were 170 and 27.5 kW kg<sup>−</sup><sup>1</sup> , respectively.

In 2016, Zhang et al. successfully prepared nanoporous NiO by dealloying Al85Ni15 alloy and calcining in air [5]. The precursor alloys were prepared by similar methods with Al-Cu [4]. The surface and pores of the sample present a nanoporous structure. The pore diameter is approximately 30–50 nm, the pore walls are composed of island-like barriers, and the width is approximately 50 nm. The entire material presents a porous frame structure with a uniform pore size. It achieved good performance in the catalytic oxidation of CO, and the active temperature of fully catalyzed oxidation of CO was approximately 340°C.

### **3.3 Nanoporous Co3O4**

In 2011, Xu et al. employed a simple fabrication method for Co3O4 nanosheets through dealloying Al-Co alloy in alkaline solutions [17]. The precursor alloy was Al95Co5, and ribbons were prepared by melt spinning. The microstructure was a hierarchical flower-like aggregate structure with the typical size at the micron scale, where each nanoflower is composed of many irregular interlaced nanoslices with thicknesses as small as 6 nm, which is a typical porous structure. The results indicated that Co3O4 nanosheets exhibited excellent catalytic activity for CO oxidation at ambient temperature, the reaction temperature for catalyzing 50% of the CO was approximately 80°C, and the temperature for completely converting the CO was 140°C. Calcination in an O2 atmosphere was essential to achieve high CO oxidation activity for these nanostructures, which allowed for the generation of active species for surface reactions. In addition, the calcination temperature significantly affected the catalytic activity; 300°C was a more favorable calcination temperature than 200 or 450°C, considering the optimum balance between the active reaction species and the surface area upon calcination. In addition, Co3O4 nanosheets showed good time-onstream catalytic stability. It is expected that many other useful metal oxide materials can be fabricated similarly. Due to the evident advantages of simple processing, nearly perfect yield, and low fabrication cost, these functional nanomaterials may lead to applications in various catalytic processes.
