Joshua D. Snyder

Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces

Giving Electrocatalysts an Edge Platinum (Pt) is an excellent catalyst for the oxygen-reduction reaction (ORR) in fuel cells and electrolyzers, but it is too expensive and scarce for widespread deployment, even when dispersed as Pt nanoparticles on carbon electrode supports (Pt/C). Alternatively, Chen et al. (p. 1339, published online 27 February; see the Perspective by Greer) made highly active ORR catalysts by dissolving away the interior of rhombic dodecahedral PtNi3 nanocrystals to leave Pt-rich Pt3Ni edges. These nanoframe catalysts are durable—remaining active after 10,000 rounds of voltage cycling—and are far more active than Pt/C. Highly active electrocatalysts are created by eroding away all but the edges of platinum-nickel nanocrystals. [Also see Perspective by Greer] 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.

Oxygen reduction in nanoporous metal–ionic liquid composite electrocatalysts

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.

Structure/Processing/Properties Relationships in Nanoporous Nanoparticles As Applied to Catalysis of the Cathodic Oxygen Reduction Reaction

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.

Activity–Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments

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.

Using Surface Segregation To Design Stable Ru‐Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments

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.

Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case

Design of active and stable Pt-based nanoscale electrocatalysts for the oxygen reduction reaction (ORR) will be the key to improving the efficiency of fuel cells that are needed to deliver reliable, affordable and environmentally friendly energy. Here, by exploring the ORR on Pt single crystals, cubo-octahedral (polyhedral) Pt NPs with different sizes (ranging from 2 to 7 nm), and 7–8 nm Pt NPs with different shapes (cubo-octahedral vs. cube vs. octahedral), we presented a surface science approach capable of rationalizing, and ultimately understanding, fundamental relationships between stability of Pt NPs and activity of the ORR in acidic media. By exploring the potential induced dissolution/re-deposition of Pt between 0.05 and 1.3 V, we found that concomitant variations in morphology of Pt(111) and Pt(100) lead to narrowing differences in activity between Pt single crystal surfaces. We also found that regardless of an initial size or shape, NPs are metastable and easily evolve to thermodynamically equilibrated shape and size with very similar activity for the ORR. We concluded that while initially clearly observed, the particle size and shape effects diminish as the particles age to the point that it may appear that the ORR depends neither on the particle size nor particle shape.

Multimetallic Core/Interlayer/Shell Nanostructures as Advanced Electrocatalysts

The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of atomic structure and compositional gradients is a guiding principle in exploiting the knowledge from well-defined materials in the design of novel class of core-shell electrocatalysts comprising Ni core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic system is found to have the optimal balance of activity and durability due to the synergy between the stabilizing effect of subsurface Au and modified electronic structure of surface Pt through interaction with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell structure exhibit high intrinsic and mass activities as well as superior durability for the oxygen reduction reaction with less than 10% activity loss after 10,000 potential cycles between 0.6 and 1.1 V vs the reversible hydrogen electrode.

Dealloying Silver/Gold Alloys in Neutral Silver Nitrate Solution: Porosity Evolution, Surface Composition, and Surface Oxides

The electrochemistry of dealloying silver/gold alloys in neutral pH silver nitrate solution to form nanoporous gold (NPG) is discussed. At pH 7, porosity evolution was observed to occur at high potentials, above that required for oxygen evolution, and within the nominal domain of the Pourbaix diagram where silver would be expected to form a passivating oxide. Electron microscopy shows that a small pore (∼5 nm) NPG is formed over a potential regime of 1.3-2.0 V vs normal hydrogen electrode, but electrochemical measurements show that the specific capacitance of samples over the same voltage range rises nearly threefold. The observations are explained in terms of residual surface oxides passivating the pores behind the dissolution front, which is itself acidified (and thus corrosive) due to an accumulation of protons associated with oxide formation and water dissociation. A model is proposed that is consistent with the electrochemical and microscopy results. This method of fabricating NPG has advantages of simplicity and safety, and the porosity formation mechanism may be extended to other systems.

Dealloying of Noble-Metal Alloy Nanoparticles

Dealloying is currently used to tailor the morphology and composition of nanoparticles and bulk solids for a variety of applications including catalysis, energy storage, sensing, actuation, supercapacitors, and radiation damage resistant materials. The known morphologies, which evolve on dealloying of nanoparticles, include core-shell, hollow core-shell, and porous nanoparticles. Here we present results examining the fixed voltage dealloying of AgAu alloy particles in the size range of 2-6 and 20-55 nm. High-angle annular dark-field scanning transmission electron microcopy, energy dispersive, and electron energy loss spectroscopy are used to characterize the size, morphology, and composition of the dealloyed nanoparticles. Our results demonstrate that above the potential corresponding to Ag(+)/Ag equilibrium only core-shell structures evolve in the 2-6 nm diameter particles. Dealloying of the 20-55 nm particles results and in the formation of porous structures analogous to the behavior observed for the corresponding bulk alloy. A statistical analysis that includes the composition and particle size distributions characterizing the larger particles demonstrates that the formation of porous nanoparticles occurs at a well-defined thermodynamic critical potential.

Kinetics of Crystal Etching Limited by Terrace Dissolution

We describe an experiment whose results are interpreted as a direct measurement of the chemical reaction kinetics of atoms being removed from high coordination crystal terrace sites, at least in the particular restricted case of electrochemical dissolution of metal alloys. The central idea is that there are binary metal alloys such as Ag/Au with the property that they can be subjected to electrochemical dissolution (corrosion) conditions where one element, the less noble component, dissolves and the remaining (minor, more noble) element diffuses on the surface, passivating all step edges and other low coordination sites. Further dissolution, particularly under conditions leading to porosity evolution, is rate-limited by dissolution from nine-coordinated terrace sites in face-centered cubic alloys. By dealloying Ag from Ag-Au alloys in ionic liquids, we have extrapolated the activation barrier for Ag dissolution from alloy terrace sites over the composition range 0-20 atom % Au.