Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement in both activity and durability. We synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals. The starting material, crystalline PtNi3 polyhedra, transforms in solution by interior erosion into Pt3Ni nanoframes with surfaces that offer three-dimensional molecular accessibility. The edges of the Pt-rich PtNi3 polyhedra are maintained in the final Pt3Ni nanoframes. Both the interior and exterior catalytic surfaces of this open-framework structure are composed of the nanosegregated Pt-skin structure, which exhibits enhanced oxygen reduction reaction (ORR) activity. The Pt3Ni nanoframe catalysts achieved a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity, respectively, for this reaction (relative to state-of-the-art platinum-carbon catalysts) during prolonged exposure to reaction conditions.

In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ Pt < Ir < Ru ≪ Os) are, in fact, the least stable (Au ≫ Pt > Ir > Ru ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.

The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O(2) + 4H(+) + 4e(-) → 2H(2)O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt-C; refs 1-7). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt(3)Ni(111) is one of the most active surfaces for the ORR (ref. 8), owing to a shift in the d-band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O(2) adsorption. However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization, especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni-Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.

The selection of oxide materials for catalyzing the oxygen evolution reaction in acid-based electrolyzers must be guided by the proper balance between activity, stability and conductivity—a challenging mission of great importance for delivering affordable and environmentally friendly hydrogen. Here we report that the highly conductive nanoporous architecture of an iridium oxide shell on a metallic iridium core, formed through the fast dealloying of osmium from an Ir25Os75 alloy, exhibits an exceptional balance between oxygen evolution activity and stability as quantified by the activity-stability factor. On the basis of this metric, the nanoporous Ir/IrO2 morphology of dealloyed Ir25Os75 shows a factor of ~30 improvement in activity-stability factor relative to conventional iridium-based oxide materials, and an ~8 times improvement over dealloyed Ir25Os75 nanoparticles due to optimized stability and conductivity, respectively. We propose that the activity-stability factor is a key “metric” for determining the technological relevance of oxide-based anodic water electrolyzer catalysts.

Porous precious metals with pore size distributions centered below 5 nm are useful for applications in electrocatalysis, sensing, and others, where the combination of high surface area and electric contact to all surface sites is advantageous. Such materials possess the virtues of nanoparticles with their good surface-area-to-volume ratio and few of their downsides, for example, there are no supports to corrode or de-adhere from and particle sintering would not lead to a reduction in surface area. A few methods have been developed to make porous precious metals with pore sizes below 5 nm. One method is templated growth, plating into the interstices of a porous parent phase that is later removed.[1] A different methoddealloying -is receiving increased attention recently because of its ease in processing.[2] One starts with a monolithic alloy in any form factor, such as bulk or thin film, and selectively electrochemically dissolves the less-noble component of the alloy. Here, we demonstrate that small fractions of Pt added to precursor Ag/Au alloys result in a new ultraporous metal upon dealloying, possessing a pore size peaked at less than 4 nm, a self-assembled core/shell structure, and remarkable stability against coarsening. The material called nanoporous gold (NPG or np-Au) is made by dealloying silver from Au/Ag alloys with compositions of Au in the 20 -40 at % range; in this way, one makes a beautiful nanoporous form of nearly pure gold with pore size in the 10-20 nm range. NPG is being used by a number of groups in a variety of important ''nano''-related applications. The thermodynamics of surface stress at the nanoscale leads to physical actuation of NPG under electrochemical potential control; [3] it is a good substrate for surface enhanced Raman spectroscopy (SERS); [4] and NPG is a good substrate on which to coat catalytically useful materials such as Pt or thiol-based self-assembled monolayers [5] (it is also unusually catalytic itself [6][7][8][9] ). Unfortunately, NPG is prone to coarsening, particularly in acidic environments, reducing its long-term functionality.[10]

We present a comprehensive experimental study of the formation and activity of dealloyed nanoporous Ni/Pt alloy nanoparticles for the cathodic oxygen reduction reaction. By addressing the kinetics of nucleation during solvothermal synthesis we developed a method to control the size and composition of Ni/Pt alloy nanoparticles over a broad range while maintaining an adequate size distribution. Electrochemical dealloying of these size-controlled nanoparticles was used to explore conditions in which hierarchical nanoporosity within nanoparticles can evolve. Our results show that in order to evolve fully formed porosity, particles must have a minimum diameter of ∼15 nm, a result consistent with the surface kinetic processes occurring during dealloying. Nanoporous nanoparticles possess ligaments and voids with diameters of approximately 2 nm, high surface area/mass ratios usually associated with much smaller particles, and a composition consistent with a Pt-skeleton covering a Ni/Pt alloy core. Electrochemical measurements show that the mass activity for the oxygen reduction reaction using carbon-supported nanoporous Ni/Pt nanoparticles is nearly four times that of commercial Pt/C catalyst and even exceeds that of comparable nonporous Pt-skeleton Ni/Pt alloy nanoparticles.

The methods used to improve catalytic activity are well-established, however elucidating the factors that simultaneously control activity and stability is still lacking, especially for oxygen evolution reaction (OER) catalysts. Here, by studying fundamental links between the activity and stability of well-characterized monometallic and bimetallic oxides, we found that there is generally an inverse relationship between activity and stability. To overcome this limitation, we developed a new synthesis strategy that is based on tuning the near-surface composition of Ru and Ir elements by surface segregation, thereby resulting in the formation of a nanosegregated domain that balances the stability and activity of surface atoms. We demonstrate that a Ru0.5Ir0.5 alloy synthesized by using this method exhibits four-times higher stability than the best Ru-Ir oxygen evolution reaction materials, while still preserving the same activity.

Recent advances in oxygen reduction reaction catalysis for proton exchange membrane fuel cells (PEMFCs) include i) the use of electrochemical dealloying to produce high surface area and sometimes nanoporous catalysts with a Pt‐enriched outer surface, and ii) the observation that oxygen reduction in nanoporous materials can be potentially enhanced by confinement effects, particularly if the chemical environment within the pores can bias the reaction toward completion. Here, these advances are combined by incorporating a hydrophobic, protic ionic liquid, [MTBD][beti], into the pores of high surface‐area NiPt alloy nanoporous nanoparticles (np‐NiPt/C + [MTBD][beti]). The high O2 solubility of the [MTBD][beti], in conjunction with the confined environment within the pores, biases reactant O2 toward the catalytic surface, consistent with an increased residence time and enhanced attempt frequencies, resulting in improved reaction kinetics. Half‐cell measurements show the np‐NiPt/C+[MTBD][beti] encapsulated catalyst to be nearly an order of magnitude more active than commercial Pt/C, a result that is directly translated into operational PEMFCs.