UCLA

Fuel cells that can directly convert the chemical energy stored in fuels to electricity are attracting increasing attention to enable a clean energy future. Compared with compressed hydrogen applied for industrialized hydrogen fuel cells, liquid fuels (e.g., methanol, ethanol or hydrazine) feature higher volumetric energy density, more convenient storage and transport, and lower cost, making them attractive alternative candidates for fuel cells. However, the anode oxidation reactions for these fuel molecules usually show higher kinetic barriers compared with hydrogen and often require noble metal based electrocatalysts, which are costly and limits the widespread adoption of the relevant technologies. Therefore, an important challenge to develop fuel cells with low the overall cost is to develop highly effective electrocatalysts with high mass activity (MA), stability, faradaic efficiency (FE) and low overpotential, as well as long lifetime.Methanol oxidation reaction (MOR) is the anode reaction of the direct methanol fuel cells. To facilitate MOR, I developed ultrathin Rh wavy nanowires with ultrahigh electrochemical surface area (ECSA ~144.2 m2/g) as the highly effective electrocatalyst with nearly 3 times of MA (722 mA/mg) compared with the previously reported Rh-based nanomaterials. More importantly, the ultrathin Rh wavy nanowires retain the advantage of the Rh-based nanomaterials to exhibit a lower the overpotential for MOR with the current peak potential at 0.61 V Vs. RHE (compared with 0.8-0.9 V Vs. RHE for Pt-based nanomaterials), leading to very high MA compared with the previously reported Rh and Pt-based nanomaterials at 0.61 V vs. RHE for MOR. Ethanol oxidation reaction (EOR) is also important for direct ethanol fuel cell application with even higher energy density, lower cost and lower toxicity compared with methanol. I designed and developed a facile solvothermal process for the synthesis of ultrathin alloy Pt3Ag wavy nanowires as highly effective EOR electrocatalysts. The alloy Pt3Ag (111) facet helps EOR by facilitating C-C bond cleavage as inspired by the previous theoretical prediction and thus contribute to higher specific activity (SA) and FE. The alloy of Ag with Pt also adjusts the electronic structure and enriches the electron density around Pt to lower the poisoning effect from carbonaceous species. Together, the resulting Pt3Ag alloy wavy nanowires deliver an ultrahigh SA of 28.0 mA/cm2 and an exceptional MA of 6.1 A/mg, far exceeding that of the benchmark commercial Pt/carbon black samples (1.1 A/mg) and higher than most of the previously reported state-of-the-art noble metal-based electrocatalysts. Moving one step further, I designed and synthesized medium entropy Au-doped PtAgRhCu alloy wavy nanowire. By simultaneously employing ligand effect, strain effect and bifunctional effect upon the introduction of Ag, Rh, Cu and Au elements, this strategy help alleviate the poisoning from the carbonaceous species. In addition, the introduction of these various elements may lead to unexpected synergistic effect (cocktail effect), resulting in further improved MA (8.43�0.40 A/mgnoble metal) for EOR with excellent long-term durability, which may be further attributed to the sluggish diffusion effect due to the enhanced entropy of the system, and thus making it a highly promising anode electrocatalysts for the alkaline direct alcohol fuel cell applications. Hydrazine also has high energy density and high potential output for fuel cell applications together with the advantage of zero carbon emission. I designed and synthesized RhRu0.5 alloy wavy nanowires as highly effective electrocatalysts for hydrazine oxidation reaction (HzOR). In addition to the ultrahigh ECSA (>100 m2/g) derived from the ultrathin wavy nanowire morphology, the alloying of Ru can greatly lower the overpotential compared with the Rh nanowires while retaining the capability to achieve the total electrooxidation of the hydrazine as confirmed from the rotating disk electrode (RDE) study, which can be attributed to the synergistic effect between Rh and Ru and the more facilitated removal of the surficial adsorbed hydrogen species. The resulting RhRu0.5 alloy wavy nanowire catalysts deliver an ultrahigh mass activity of 60.4�6.2 A/mg at 0.20 V vs. RHE along with high geometric current density and excellent long-term performances, demonstrating outstanding potential for alkaline direct hydrazine fuel cells.