A successful working model for nanoporosity evolution during dealloying was introduced 15 years ago. Since that time, the field has rapidly expanded, with research groups from across the world studying dealloying and dealloyed materials. Dealloying has grown into a rich field, with some groups focusing on fundamentals and mechanisms of dealloying, other groups creating new porous metals and alloys, and even more groups studying their properties. Dealloying was originally considered only in the context of corrosion, but now it is considered a facile self-organization technique to fabricate high-surface-area, bicontinuous nanoporous materials. Owing to their high interfacial area and the versatility of metallic materials, nanoporous metals have found application in catalysis, sensing, actuation, electrolytic and ultracapacitor materials, high-temperature templates/scaffolds, battery anodes, and radiation damage–tolerant materials. In this review, we discuss the fundamental materials principles underlying the formation of dealloyed materials and then look at two major applications: catalysis and nanomechanics.

The activity for an electrocatalyst for the oxygen reduction reaction (ORR) is most often assessed using bare metal electrodes in direct contact with an aqueous electrolyte. This architecture allows all reactants and products to have equivalent geometric access to the catalyst surface, but this does not always lead to an optimal reaction environment. By adding an intermediate phase as a layer between the catalyst and the electrolyte, diffusive driving forces can be engineered into the system to force reactants to the catalyst and products away from it, increasing the overall system activity. For instance, previous research explored nanoporous Ni/Pt electrodes encapsulated with the ionic liquid [MTBD][beti]. The high oxygen solubility in this ionic liquid was thought to explain the nearly doubled increase in the composite electrocatalyst activity, but it is possible that other ionic liquid properties (water solubility, oxygen diffusivity, ionic conductivity, viscosity) might also be affecting ORR activity. In this work, we surveyed a number of ionic liquids in nanoporous NiPt/ionic liquid composite catalysts with an eye toward clarifying to what extent the physical properties of the IL have on the activity of the composite ORR catalyst. Overall, we find the oxygen solubility and water solubility most strongly affect the decrease in ORR overpotential. The cathodic oxygen reduction reaction (ORR) is the most inefficient component of hydrogen fuel cells, and implementation of this technology will require better catalysts for this reaction. The 4-electron ORR has been studied for many decades. It is a complex catalytic reactioninvolving mass transport of oxygen molecules and protons to a catalytic metal surface, and (in the absence of an aqueous solution) expulsion of product water away from this surface. Both theoretical and experimental work has led to many improvements in Ptbased catalysts for the ORR, from geometric alterations of the catalyst increasing the active surface area through nanoparticle shape control, 1 to porosity formation, 2 to alloying Pt with transition metals. 3While these adjustments have improved activity over commercially available catalysts, it remains unclear that these increases are sufficient for the adoption of fuel cells into mainstream automotive applications. 4 Previously explored in our group is a composite catalyst for oxygen reduction using a dealloyed nanoporous nickel-platinum catalyst (npNiPt) impregnated with an ionic liquid (IL) whose properties bias the reaction kinetics for the ORR to completion. This approach increases the aggregate catalytic activity of the composite by a multiplicative factor of the base np-NiPt catalyst activity.5 This strategy was recently used by Stamenkovic, et al. to make a composite Ni/Pt nanoframe + IL catalyst that has resulted in a 36-fold increase in activity for the ORR compared to neat Pt. 6 However, fundamental electrochemical principles and measurements have not been made for the porous catalyst + IL composite, and it is the purpose of this ...

Electroreduction of small molecules in aqueous solution often competes with the hydrogen evolution reaction (HER), especially if the reaction is driven even moderately hard using a large overpotential. Here, the oxygen reduction reaction (ORR) was studied under proton diffusion-limited conditions in slightly acidic electrolytes: a model system to study the relative transport kinetics of protons and reactants to an electrocatalyst and the relationship between transport and catalytic performance. Using dealloyed nanoporous nickel-platinum (np-NiPt) electrodes, we find the hydrogen evolution reaction can be completely suppressed even at high overpotentials (-400 mV vs RHE). In addition, the mechanism of oxygen reduction can be changed by using buffered versus unbuffered solutions, suggesting the reaction selectivity is associated with a transient rise (or lack thereof) in the interface pH at the np-NiPt surface. Independently controlling reactant transport to electrocatalyst surfaces at high overpotentials exhibited a surprisingly rich phenomenology that may offer a generalizable strategy to increase activity and selectivity during electroreduction reactions.

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A highly efficient flow cell for sequential electrolysis containing two complete electrochemical cells, capable of generating reactive species at the upstream working electrode and transporting them to the downstream working electrode, is demonstrated. Deconvolution of the intermixed electrode circuits is accomplished through analysis of the inherent resistance of the electrolyte, which allows for precise and independent control of the electrochemical potential at each electrode without altering concentrations of supporting or background electrolyte species. Sequential electrolysis involving oxidation of hydrogen and reduction of the generated protons downstream is demonstrated at nearly 100% efficiency on Pt-decorated dealloyed porous Nb catalysts. The conversion efficiency of the catalysts is discussed in terms of their geometries and active surface composition, elucidating strategies for use of sequential electrolysis cells for fundamental and applied studies.