Abstract:
Rapid advances in technology and industrialization have resulted in catastrophic climate change and contamination of natural resources. Clearly the solution for these problems involves using clean alternative energy sources and efficient means for removal of pollutants. Electrocatalysis plays a key role and by increasing its efficiency can reduce the cost of current technologies, which is one of the major barriers for large-scale implementation. Understanding the factors affecting heterogeneous electrocatalysis is fundamental to improving the efficiency and reliability of many devices used in renewable energy applications. Therefore, this work is focused on enhancing the efficiency of heterogeneous electrocatalytic processes in two areas: a) The electrocatalytic reduction of oxygen and the oxidation of methanol which are crucial for the development of more efficient fuel cells, and b) The electrocatalytic removal of nitrate from the environment to revitalize fresh water.
Self-assembled monolayers (SAMs) are employed as active templates for synthesis of Pt particles using chronoamperometric (CA) electrochemical deposition. We evaluated the effect of thiol terminal groups with various degrees of hydrophobicity (CH3, COOH and OH) and their chain lengths (C6, C10/C11 and C16) on Pt particle growth. The results suggest that the deposition mode (over potential deposition (OPD) and under potential deposition (UPD)), size and distribution of deposited particles are strongly affected by the terminal groups of thiols and their hydrophilicity/hydrophobicity. Methyl terminal groups lead to the expected OPD deposition, but surprisingly increased the particle count and decreased particle sizes with increasing thiol length. Both hydrophilic terminal groups demonstrated UPD and as expected decreased the particle count and increased particle sizes with increasing the thiol chain length. The results suggest that thiol motion and hydrophobicity significantly influence particle growth behavior.
We then controlled the particle surface morphologies by varying two growth conditions, pH and deposition potential using C16 carboxylate thiol (HOOC(CH2)15SH) since it forms the most consistent and dense SAM on gold as an active template for Pt deposition. Both approaches demonstrated a control over the morphology of the particles but due to simplicity of the process, applied potential was used in all studies present in this thesis. The efficiency of electrocatalytic oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) are then evaluated as a function of the morphology of Pt particle based on surface and mass activity and turn over number. Four different morphologies, “spherical”, “dendrite”, “spiky” and “clusters”, were synthesized reproducibly based on applied voltage. The results demonstrated a significant
improvement in catalytic activity of Pt for both ORR and MOR for spiky morphology when compared to the others. While, the cluster particles present a very poor electrocatalytic performance for these two processes despite their smaller size.
The methodology we developed for controlled deposition of various Pt morphologies was then used to evaluate the influence of morphology of Cu on electrocatalytic nitrate reduction reaction (NRR). Initially a methodology for forming a consistent and dense SAM using HOOC(CH2)15SH on Ni was developed to act as an active template for Cu deposition. Again, changing the voltage for electrochemical Cu deposition resulted in formation of particles with different morphologies in a controlled fashion. The electrocatalytic efficiency and selectivity of these different morphologies for NRR was then investigated. Surprisingly, the morphology of the catalyst influenced the mechanism of NRR. Our results demonstrated that by increasing the Cu deposition potential, resultant particles tend to catalyze the reduction of nitrate towards ammonia, while lower Cu deposition potential produces particles with tendency to drive the reaction towards nitrite production.
The results from this thesis have demonstrated the impact of morphology on the efficiency, mechanism and selectivity of electrocatalytic reactions. This can open new avenues for more efficient hydrogen, and methanol fuel cells as well as water treatment devices.