Application of Novel Nanoscale Materials and Advanced Electrochemical Techniques in Heterogeneous Catalysis and Fabrication of Combinatorial Material Libraries by Bipolar Electrodeposition

Abstract

The advancements in the electrochemical technologies combined with the introduction of novel materials makes our lives a lot better. This work has two major divisions: (1) development of better electrocatalysts for fuel cells and (2) application of bipolar technique in combinatorial material library synthesis. Chapters 1 gives a brief introduction of polyoxometalates, an inorganic molecule capable of conducting multi-electron, proton and oxygen and so used as electrocatalyst for many redox reaction. It also gives a short account on carbon nanotubes (CNTs), an excellent supporting material for electrocatalysts, with high surface area, high electrical conductance and excellent mechanical strength. Finally it deals with the methods to establish lateral potential gradient on a conductive surface. The methods mainly include bipotentiostat and bipolar technique. Lateral potential gradient helps to make the combinatorial materials library on a single substrate. Chapter 2 deals with synthesis of Dawson and transition metal substituted Dawson type polyoxometalates, their characterization, redox properties and electrocatalytic activity towards oxygen reduction reaction (ORR). Results show that all these POMs are hydrolytically stable at low pH and electrocatalytically active towards ORR in 0.1M perchloric acid medium. The Dawson compounds follow the following trend in ORR electrocatalytic activity: Dawson , Co-Dawson < Ru-Dawson < Fe-Dawson. Chapter 3 deals with bimetallic synergy between transition metal substituted Dawson type POMs and noble metals electrodes such as gold, palladium and platinum for ORR. In this case the transition metal in POM helps to cleave the O-O bond in dioxygen and the noble metal surface is good at reducing the cleaved oxygen atoms to water. The bond strength between the transition metal substituted in Dawson and oxygen atom should be high enough to cleave O=O bond of dioxygen and at the same time it should allow the transfer of cleaved oxygen atom to the noble metal surface. So, the ORR potential shift on gold electrode with transition metal substituted POM taken in 0.1M perchloric acid medium depends on the free energy of formation of bulk metal oxide of corresponding substituted transition metal in the POM. The maximum ORR potential shift obtained was about +200 mV compared to pure gold surface. It occurred with 0.2 mM Ru-Dawson in 0.1M perchloric acid medium. With palladium electrode Dawson and Fe-Dawson gave a negative ORR shift. Ru- and Co- Dawson gave positive ORR potential shift. The maximum ORR potential shift obtained on Pd was +100 mV with 0.2 μM Co-Dawson. On platinum surface all Dawson type POMs except Co-Dawson gave negative potential shift due to surface poisoning. Co-Dawson at 0.2 μM gave a positive shift of about +25 mV compared to pure Pt. Less adsorbing Keggin type POM with Co substitution gave a maximum shift of about +54mV on Pt surface which is comparable with other bimetallic electrocatalysts. Chapter 4 deals with the ORR electrocatalytic activity of oxidized MWNT and oxidized MWNT-Co-Dawson adduct prepared by sonication method. Compared to glassy carbon, oxidized MWNT gave a 200 mV positive shift in ORR E1/2 potential. Co-Dawson treated oxidized MWNT gave another 200mV positive shift. Though the absence of Co-Dawson redox peaks in the Co-Dawson treated oxidized MWNT under deoxygenated condition indicates the decomposition of Co-Dawson during the process of sonication with oxidized MWNT, due to the significant positive shift in ORR potential, we further studied the kinetics of ORR on the oxidized MWNT-Co-Dawson adduct. 12 hr sonication with Co-Dawson has doubled the ORR kinetic current of the oxidized MWNT. Chapter 5 deals with the formation and characterization of one-dimensional chemical composition gradients of CdS on Au surfaces using bipolar electrodeposition. When an external electric field is applied across an electrically floating Au electrode immersed in a bipolar electrochemical cell, a position-dependent interfacial potential difference is generated along the length of the Au. This potential gradient can be used to induce variations of chemical composition within thin films electrodeposited onto the Au bipolar electrode (BPE). Thin films formed by bipolar electrodeposition represent continuous one-dimensional solid-state material libraries and were screened using resonance Raman microscopy and Auger electron spectroscopy. As predicted from simple thermodynamic considerations, we observed three distinct deposition zones scanning from the cathodic pole to the midpoint of the BPE: (i) CdS+Cd, (ii) stoichiometric CdS, and (iii) elemental S. Bipolar electrodeposition can be used to generate material libraries rapidly and without direct electrical contact to the substrate using extremely simple instrumentation. Chapter 6 gives the summary of this research work and a few directions for further studies.