Synthesis of noble metal based nanocatalysts for PEM fuel cell applications

Electrocatalysts for energy conversion and power generation systems such as fuel cells are mostly based on Pt-group metals. Because Pt is a strategic metal, is expensive and scarce, any reduction of its load in catalyst electrodes can result in cost-reduction of fuel cells and electrodes for other Pt-catalyzed reactions, e.g. for the hydrogen evolution reaction (HER). This thesis is devoted to the study of 0D and 1D noble-metal-based nanostructures with reduced Pt-load for the electrooxidation of formic acid and methanol, and the HER. The effect of different parameters on the electrocatalytic activity is examined with the aim of reducing the mass loading of platinum-group metals while boosting performance of catalysts. 0D nanoparticles (NPs) are synthesized using sonochemistry in aqueous solutions of noble metal ions and mixtures thereof. With the help of a simple laboratory ultrasonic device nanoparticles of AuPd, and PtPd are deposited on nanocarbon and TiN substrates. The mechanisms of noble metal NP formation on the aforementioned substrates are investigated and discussed in terms of reduction of noble metal ions by organic radicals that form at cavitation- substrate interface. After microscopic and structural characterization, the supported NPs are subsequently used for the electrooxidation of formic acid. It is shown that the electrocatalytic performance very much depends on the nanoalloy composition with AuPd22 nanoalloy exhibiting highest activity of 400 A/g, and a direct reaction path due to the presence of gold that can prevent catalyst poisoning with CO. Crossing over the dimensionality, 1D nanostructures of different morphologies and chemistries were processed with the aim to boost electrocatalytic activities still more. Self-standing layered Au/Pd/Au and Au/Pd nanorods with different Pd-layer thickness were synthesized using sequential electrodeposition in porous anodic aluminum oxide (AAO) template thin films. It is shown that the electrooxidation of formic acid very much depends on the Pd-layer thickness with thinner layers, down to 15 nm, being most active. This is explained in terms of the formation of near interface AuPd alloys via Pd-diffusion into Au, based on the shift of the PdO reduction peak toward more noble values with decreasing Pd thickness. All the nanostructures show a direct path, suggesting no poisoning of Pd with CO or other carbonaceous species. Density functional theory (DFT) calculations were conducted, and provide insights into the role of Au as an oxidant of CO adsorbed on Pd in the near-interface alloys. Pt/C is the best and most widely used catalyst for direct methanol fuel cells (DMFCs) that enjoy an ever-increasing attention for energy conversion. In this thesis, the Pt-loading is reduced first by adopting a nanotube morphology that is realized via electrodeposition at negative voltages, and second by alloying Pt with Pd. These approaches permitted the increase in the electrocatalytic activity to 932 A/gPt (in 0.5 M H2SO4) for an extremely low Pt content of 10 at.% as well as in the electrochemical surface area (ECSA) to 1358 m²/g. These results are at the upper end of what was published so far, and were achieved via the enormously higher surface area in contact with the electrolyte (nanotube morphology) and the ligand effect (alloying Pt with Pd). The results are rationalized using DFT calculations which show that CO preferentially adsorbs on Pd, thus keeping the active Pt sites free from poisoning species. Hydrogen production via water electrolysis using wind and solar power is a promising way for energy storage from renewables. Hydrogen is produced from aqueous electrolytes via HER in which noble metal electrocatalysts, prominent among them is Pt, play a key role. In this thesis, Pt and Pd nanostructures, monolithic and supported on Au-NRs, are processed either via electrodeposition or magnetron sputtering, and compared as to their electrocatalytic activity. While the electrodeposited and sputtered monolithic and layered nanostructures show only a moderate activity towards HER, sputtered Pt and Pd on Au-NRs are characterized by a largely higher activity. This is explained by the formation of finely distributed Pt(Pd)-NPs on the entire surface of the Au-NRs thus maximizing the interfacial interaction area between Pt(Pd) and Au. Surprisingly, sputtered Pd on Au-NRs is shown to surpass sputtered Pt on Au-NRs (Pt is known to be the best catalyst for HER). For similar nanostructures, the gravimetric current density measured in 0.5 M H2SO4 is 313 mA/mg.cm2 at -0.1 V for Pt against 480 mA/mg.cm2 for Pd. The performance could be boosted still more by choosing longer Au-NRs as supports for Pd. The mechanisms of HER are discussed using Tafel-Plots and Tafel slopes. The sputtered Pd/Au nanostructure is characterized by the lowest Tafel slope suggesting a fast Volmer discharge step followed by a Heyrovsky recombination mechanism.


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