Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Nanoporous Gold for Energy Applications

Research activities using nanoporous gold (NPG) were reviewed in the field of energy applications in three categories: fuel cells, supercapacitors, and batteries. First, applications to fuel cells are reviewed with the subsections of proof-of-concept studies, studies on fuel oxidations at anode, and studies on oxygen reduction reactions at cathode. Second, applications to supercapacitors are reviewed from research activities on active materials/NPG composites to demonstrations of all-solid-state flexible supercapacitors using NPG electrodes. Third, research activities using NPG for battery applications are reviewed, mainly about fundamental studies on Li-air and Na-air batteries and some model studies on improving Li ion battery anodes. Although NPG based studies are the main subject of this review, some of meaningful studies using nanoporous metals are also discussed where relevant. Finally, summary and future outlook are given based on the survey on the research activities.

In situ carbon-supported titanium dioxide (ICS-TiO2) as an electrode material for high performance supercapacitors

Supercapacitors have attracted significant attention in the last few years as they have the capability to fulfill the demand for both power and energy density in many energy storage applications. In this study, an in situ carbon-supported titanium oxide (ICS-TiO₂) electrode has been prepared using sucrose and TiO₂ powder. The ICS-TiO₂ powder was prepared by slipcasting, followed by the annealing of the TiO₂ slurry. Sucrose was added to the TiO₂ slurry as a soluble carbon source, and was converted into carbon at 600 °C then coated on the TiO₂ particles. The morphological and structural evolution of the electrode was investigated by FEG-SEM, FEG-TEM, XRD, BET, FTIR, XPS and Raman spectroscopy. The electrochemical characterization of ICS-TiO₂ demonstrated that this material exhibits an efficient value of specific capacitance (277.72 F g^-1 at 25 mV s^-1) for charge storage. ISC-TiO₂ also exhibits a specific capacitance of 180 F g^-1 at 2 A g^-1 in a 1 M Na₂SO₄ aqueous electrolyte. The results suggest that ICS-TiO₂ can be utilized as a high-performance electrode material for supercapacitors with desirable electrochemical properties.