Efficient Synthesis of Noble-Metal-Based Metallic Hydrogels/Aerogels and Their Electrochemical Applications
Authors: Shi Qiurong , Zhu Chengzhou , Du Dan , Lin Yuehe 
Published: February 2020

Metallic hydrogels/aerogels are highly porous materials with ultralow density, large surface area, and hybrid interlinked pores. These novel nanostructures not only combine the physiochemical properties of nanomaterials from the macroscale, but also possess the fascinating properties in term of their various size, composition, and morphology effect from the atomic scale. Generally, there are two different strategies for synthesizing noble-metal-based hydrogels/aerogels, i.e., self-assembly and in situ reduction and subsequent fusion. However, rational design and synthesis of self-supported noble-metal-based hydrogels/aerogel is still challenging and there are some critical issues that should be addressed to optimize their morphological and compositional characteristics by means of more efficient synthesis routes and expands their practical applications. First, the occurrence of gel formation necessitates the concentration of pristine colloidal dispersion in most cases and formation of these assembled aerogels usually takes a several days to weeks. Second, due to the specific properties of metal nanoparticles, some noble metal hydrogels (e.g. AuPt) are not available until now for using the above-mentioned two traditional strategies. Thirdly, introduction of non-precious metals such as Fe, Co, Cu, Ni and so on to noble-metal-based hydrogels/aerogels has been rarely studied to construct promising bi- or multi-metallic hydrogel systems and further broaden their potential applications.
Here, we report a series of noble-metal-based metallic hydrogels/aerogels including the efficient synthesis methods, rational designed morphologies, structures, compositions and their applications to shed light on the progress we have achieved. At first, we synthesized Au@Pt3Pd ternary metallic hydrogels (TMHs) through the combination of the above-mentioned two strategies that only takes 4 hrs at 35 °C. The as-synthesized Au@Pt3Pd TMHs are assembled from building blocks with dendritic and core-shell structured nanoparticles with size of about 24.5 nm. They exhibited enhanced activity and stability toward oxygen reduction reaction due to the electronic and geometric effect, and intrinsic support-less 3D hybrid pore systems. In order to further enhance the synthesis efficiencies of metallic hydrogels, we developed a more universal and facile synthesis strategy of CuM (M=Au, Pd, Pt) metallic hydrogels/aerogels from the one step in situ reduction of metal precursors with NaBH4 through enhancing the gelation kinetics in aqueous solution without any surfactants. The importance of this work lies in that we successfully introduced non-precious metal into the noble-metal hydrogel systems. Besides, the synthesis strategy is time-saving, surfactant-free, readily for scale up, which endows the noble-metal-based hydrogels with more merits for application in electrocatalysts. The as-obtained PdCu metallic hydrogels achieved a mass activity of 3.5 A/mg toward ethanol electro-oxidation, which is more than two times higher than commercial Pd/C. Employing the similar approach, for the first time, we successfully synthesized AuPt bimetallic hydrogels/aerogels, which previously remains a challenge in the field of metallic hydrogels. The composition optimized AuPt5 metallic hydrogel achieved enhanced stability and mass activity of 0.7 A/mg toward electro-oxidation of methanol, which is 3.5 times higher than commercial Pt/C. In order to take full advantage of the metallic hydrogels and also further boost the catalytic performances, we modified the composition and structure of metallic hydrogels from the atomic scale and synthesized Pd ensemble anchored Au2Cu metallic hydrogels (denoted as Au2Cu@Pd) employing core-shell structured Au2Cu hydrogel as the backbone via galvanic displacement reaction. The as-obtained Au2Cu@Pd achieved an outstanding catalytic activity toward ethanol oxidation with mass activity reached to 22.1 A/mg, and excellent stability of maintained 90 % of original mass activity after 300 potential cycles.
All in all, we have developed the efficient synthesis strategy of noble-metal-based metallic hydrogels/aerogels including decreasing the gelation time, eliminating the surfactant and so on. Besides, we introduced the non-precious metals into the hydrogel systems, which could further decrease the price and improve the electrochemical performances through synergistic effect. What’s more, we successfully synthesized AuPt bi-metallic hydrogels for the first time. Finally, we rationally modified the metallic hydrogels from the atomic scale and achieved the excellent electrocatalytic performances toward small molecule electro-oxidation. Our strategies opened an avenue for the synthesis of mono-, bi-, and multi- metallic aerogels with addition of non-noble elements, which is of great significance in boosting the commercialization of electrochemical catalysts not only for energy conversion and storage devices, but also for optics, sensors, electronics and so on.