Designing of electrocatalyst towards sustainable energy conversion elicited by microelectrochemical visualization

Abstract

Oxygen, the most abundant element on earth is vital for the survival of majority of life that exists on this planet and also supports a comfortable life in this era of global development by its eminent implications towards energy conversion and storage. This has spurred interest amongst the researchers to explore oxygen chemistry wherein a pertinent area recognized has been oxygen reduction reaction which lies at the heart of a flock of devices including fuel cells, metal-air batteries, HCl electrolysis, and oxygen sensors to name a few. Nevertheless, it is a formidable challenge to break the O-O bond by supplying electrons and protons even over a catalyst surface and this forms the basis of the present dissertation. Platinum is one of the most popular catalysts widely employed although rare and expensive but it is also challenged by kinetic and stability issues beyond cost and availability. Therefore, presently there is a grave demand of improvising over this process where alkalinity comes as a respite opening up new avenues for designing varied non-precious catalysts and even escalating the kinetics. Considering these intriguing aspects three major classes of non-platinum catalysts have been explored namely semi-precious and transition metals followed by non-metallic alternatives. Initially in Chapter 3, silver has been explored towards O2 reduction as a non-platinum group metal exhibiting high stability in alkaline medium. However, keeping in mind sintering of metal nanoparticles upon prolonged activity it was tethered to a nitrogen containing carbon support by electrodeposition to induce Ag-N interaction. This favored the catalyst on both activity and stability grounds with a nearly 2% metal loading. A second approach to avoid sintering was to eliminate metallic component with the inorganic one achieved by designing silver phosphate (Ag3PO4). This was synthesized at room temperature in two morphologies viz. porous sphere and cubes giving profound activity in comparison to Pt/C. Besides, it was able to stand the test of prolonged potential application even in the presence of high concentration of methanol. Chapter 4 further moves onto evaluating the competitive oxygen reduction activity of earth abundant transition metals where the first section focusses upon tungsten oxide supported over mesoporous carbon. The nanoclusters of WOx provided active catalytic centers while mass transport and accessibility were enhanced by mesoporosity. Next section centered around manganese oxide (Mn2O3), containing the 12th most abundant element in the earth’s crust. Rod-like Mn2O3 were synthesized under controlled precipitation conditions of a carbonate precursor. Their ORR activity was subsequently investigated by advanced analytical techniques like electrochemical quartz crystal microbalance (EQCM) and laser-induced current transient (LICT) measurements to gain significant mechanistic insights. The last section in the sequence went a step ahead in exploring a completely new class of ORR catalyst i.e. manganese tungstate (MnWO4) which was benefited by the metal-to-metal charge transfer from Mn+2 to WO4 - moiety. This electronic redistribution supported over a conducting carbon was utilized by optimizing its morphology towards performing efficient ORR and in the process sketching a structure-activity relationship. Investigating a metal-free catalyst towards ORR was a major leap elaborated in Chapter 5. The synthetic protocol required polymerizing pyrrole over a softtemplate capable of self-removal during preparation without posing any harm to the resultant catalyst ensemble. Varied pyrolytic conditions accorded for differential morphology and nitrogen content into the graphitic carbon network. This deliberately introduced heteroatom thereby inducing electronic anisotropy in the resultant conductive framework which accounts for the active sites responsible for performing oxygen reduction. The process was followed both theoretically as well as electrochemically to obtain insight into the mechanistic pathway. This was furthered by visualizing the active site across the coated sample using a scanning electrochemical microscope (SECM) giving a good account of its distribution and competitiveness. Besides the onset, the ORR pathway was traced by a specialized 4-probe multiple pulse chronoamperometry at the ultramicroelectrode tip. Chapter 6 explores another aspect of a low temperature alkaline fuel cell where emphasis has been laid towards investigating aqueous alkaline borohydride as an anodic fuel. The energy quotient of borohydride is unmatchable theoretically but challenged by the complex 8 electron oxidative requirement. An initial attempt towards anodic catalyst development capable of performing borohydride oxidation at the surface of a carbon-based catalyst was explored in the first section. Although striking results were obtained considering an unconventional attempt of avoiding metallic component but it suffered from large overpotential requirement to facilitate the oxidation process. This sparked interest in the fundamentals of highly intricate borohydride oxidation which was explored in the second section at the tip of gold ultramicroelectrode. Interesting aspects contradicting the classical notion were observed but were in line with the recent in-situ spectroelectrochemical analysis. This was achieved by performing fast scan voltammetry of the order of thousand volts per second enabling redox capture of short-lived unstable species which was also supported by another uncommon electroanalytical technique termed differential normal pulse voltammetry (DNPV). The last section benefiting from previous results proposes a new cobalt tungstate (CoWO4) catalyst which gave promising results abating the overpotential issues which arose in the first section. Besides overpotential reduction, high oxidation currents were achieved with nearly 7 electron transferred paving way for further catalyst development based on such directed designing.