UC Berkeley

Increasing fossil fuel consumption and CO2 emission have raised great concerns about our energy and environment future. In order to solve these problems, on the one hand, it is critical to develop alternative, renewable energy sources, such as fuel cell powered by H2, on the other hand, it is also important to reduce the existing CO2 into value-added products, such as fuels and fine chemicals. Electrocatalysis using nanomaterials plays an essential role in completing these two tasks by transforming the reactants into the target products efficiently along with the energy conversion process. The activity, stability, and selectivity of nanocatalysts highly depend on their structures, which require careful and rational design to achieve desired properties. Therefore, this dissertation focuses on the development of multiple effective strategies to improve the performance of different nanocatalysts for either fuel cell or CO2 reduction reaction (CO2RR) electrocatalysis.

First, post-synthesis treatment has been recognized as a key step to tailor the catalytic behavior of Pt-based alloys. In the second chapter, we present the effects of catalyst processing on the electrocatalytic property of Pt-Ni nanoframes for cathodic oxygen reduction reaction (ORR) in fuel cell. The Pt-Ni nanoframes are made by corroding the Ni-rich phase from solid rhombic dodecahedral particles. Among the three different corrosion procedures, electrochemical corrosion leads to the highest initial specific activity by retaining more Ni in the nanoframes. However, the high activity gradually goes down in a subsequent stability test due to continuous Ni loss and concomitant surface reconstruction. In contrast, the best stability is achieved by a more-aggressive corrosion using oxidative nitric acid. Although the initial activity is compromised, this procedure imparts a less-defective surface, and thus, the specific activity drops by only 7% over 30,000 potential cycles. These results indicate a delicate trade-off between the activity and stability of Pt-Ni nanoframe electrocatalysts.

Second, controlled synthesis of nanoparticles with optimal morphology and composition is crucial to promoting their catalytic performance. In the third chapter, we demonstrate the integration of highly open nanoframe morphology and catalytically active Pt-Co composition to develop Pt-Co nanoframes. Their ORR mass activity in acidic media is as high as 0.40 A mgPt-1 initially and 0.34 A mgPt-1 after 10,000 potential cycles at 0.95 V versus reversible hydrogen electrode (VRHE). Moreover, their mass activity for anodic methanol oxidation reaction (MOR) in alkaline fuel cell is up to 4.28 A mgPt-1 and is 4-fold higher than that of commercial Pt/C catalyst. Experimental studies indicate that the weakened binding of intermediate carbonaceous poisons contributes to the enhanced MOR behavior. More impressively, the Pt-Co nanoframes also show excellent stability under long-term testing, which could be attributed to the negligible electrochemical Co dissolution.

Third, introducing a second material, such as the metal-organic framework (MOF), is a promising strategy to add catalytic functions beyond metal nanoparticles. In the fourth chapter, we demonstrate the combination of Pt-Ni nanoframe and zeolitic imidazolate framework-8 (ZIF-8), which is a special MOF, into an individually encapsulated frame-in-frame structure. Via surface functionalization, the Pt-Ni nanoframe is first embedded in ZIF-8 to achieve a single core-shell structure, as evidenced by the three-dimensional tomography. The growth trajectory of such frame-in-frame nanocomposite is tracked, revealing that ZIF-8 first nucleates in the solution, then attaches to the surface of the nanoframe, and finally grows to capture the entire nanoframe, enabling the one-in-one encapsulation. Next, by utilizing ZIF-67 as the sacrificial layer, the Pt-Ni nanoframe is further solely encased in ZIF-8 to form a single yolk-shell structure, which has a cavity between the core and the shell. The obtained frame-in-frame structures have potential applications in size-selective or tandem catalysis to produce fine chemicals.

Fourth, long-range atomic ordering in nanocrystals holds the promise of unique catalytic properties for many reactions. In the fifth chapter, we report the preparation of Cu3Au intermetallic nanowires by using Cu@Au core-shell nanowires as the precursors. With appropriate Cu/Au stoichiometry, the Cu@Au core-shell nanowires are transformed into fully ordered Cu3Au nanowires under thermal annealing. Thermally-driven atomic diffusion, which is facilitated by the abundant twin boundaries, accounts for the ordering process. The resulting Cu3Au intermetallic nanowires have uniform and accurate atomic positioning in the crystal lattice, which enhances the nobility of Cu. No obvious copper oxides are observed in fully ordered Cu3Au nanowires after annealing in air at 200 oC, a temperature that is much higher than those observed in Cu@Au core-shell and pure Cu nanowires. The acquired Cu3Au intermetallic nanowires are promising candidates for either ORR or CO2RR electrocatalysis.

Fifth, covalently bonded surface ligands often block the active metal sites and limit the reactivity of nanocluster catalysts. In the sixth chapter, we investigate the ligand removal process for Au25 nanoclusters using both thermal and electrochemical treatments, as well as its impact on the CO2 electroreduction to CO. The Au25 nanoclusters are synthesized with 2-phenylethanethiol as the capping agent and anchored on sulfur-doped graphene. The thiolate ligands can be readily removed under either thermal annealing at >180 oC or electrochemical biasing at <-0.5 VRHE. However, these ligand-removing conditions also trigger the structural evolution of Au25 nanoclusters concomitantly. The thermally and electrochemically treated Au25 nanoclusters show enhanced activity and selectivity for the electrochemical CO2-to-CO conversion than their pristine counterpart, which is attributed to the increased exposure of undercoordinated Au sites on the surface after ligand removal.