In this review, we extensively covered different types of PNMNs, ranging from ordered to nonordered porous nanostructures. From the perspective of the synthetic approaches, a broad range of PNMNs, such as aerogels, nanofoams, hollow structures, and highly branched nanomaterials, can be rationally designed. Based on the advantages provided by the attractive and unique properties of these PNMNs, their electrochemical applications with an emphasis on fuel cells and electrochemical sensors and biosensors are highlighted. These fascinating PNMNs provide a wealth of opportunities in multidisciplinary environments for promoting the rapid development of different research fields.
In spite of significant progress made in the synthesis of porous nanostructures, there are still many challenges ahead in the development of novel PNMNs with functional applications. Because the properties of the PNMNs largely depend on the size, shape, composition, and structure of their building blocks, precise control of these factors is necessary to improve their potential applications. To make further progress, a reliable method for synthesizing high-quality PNMNs with defined structures and properties is highly desirable. In particular, few reports have demonstrated the production of well-designed PNMNs with building blocks that are rich in high-index facets. With regard to the composition, extensive investigations have been focused on the controlled synthesis of bimetallic/multimetallic systems and their related applications, showing synergistic effects between the compositions and the enhanced by 3D porous nanostructures. For example, significant contributions have been made to the construction of the ordered PNMNs using hard templates. However, most of the ordered porous nanostructures are based on monometallic systems, while multimetallic nanostructures are rarely demonstrated. For the new emerging noble metal aerogels, the effects of composition, especially for the introduction of non-noble metals and the new synthetic methods, should be considered simultaneously. A better understanding of PNMN growth mechanisms is critical. The salient experimental and theoretical advances should also bring new avenues for probing the formation mechanism of the PNMNs constructed through different synthetic protocols.
With respect to electrochemical applications, PNMNs with hierarchically porous nanostructures hold great promise. The effect of integrated parameters of PNMNs, such as pore size and volume, the structure and composition of ligament, on the performance of the constructed electrochemical devices should be well investigated. Using highly active electrocatalysts for fuel cell and electrochemical sensors and biosensors has attracted increasing attention from the electrochemists community for the fabrication of superior electrochemical devices. In the case of fuel cells, flexibly controlling the pore structure and the surface engineering, including size, shape, composition and high-index facets of the PNMNs should be addressed in order to produce new electrocatalysts with low cost, high activities, and good durabilities. Another interesting aspect is that for electrochemical sensors and biosensors, it is necessary to rationally design and synthesize functional PNMN-based materials for fabricating novel electrochemical sensors with high sensitivity and specificity. Moreover, researchers should also focus on the porous hybrid systems involving carbon and metal oxides. These porous hybrids are expected to provide additional properties and synergistic effects, which are most likely to be exploited in different electrochemical applications involving multifunctional nanomaterials. With the development of new material fabrication approaches and characterization techniques, it is anticipated that major advancement in PNMNs and their enhanced applications in electrochemistry will emerge in the future.
