Non-Precious Metal Based Electrocatalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells and Electrolyzers.

摘要

The oxygen reduction reaction (ORR) is a key process in various electrochemical energy conversion devices such as fuel cells and metal batteries as it enables CO2-free electrical energy generation. One of the major challenges in these devices is the sluggish kinetics of ORR and thus the need for stable and highly active electrocatalysts. The currently utilized catalytic materials are based on precious group metals (PGM), including platinum, rhodium, or silver. Although the PGM-based catalysts are highly active and reasonably stable under harsh acidic fuel cell conditions, the PGM-systems contribute to high cost of the energy conversion device. This is further aggravated by the high sensitivity of the PGM-catalysts to the presence of small amounts of impurities in the real world environment causing performance decay. These challenges pushed researchers to look for a cost-effective and highly active alternate catalyst materials based on non-precious group metals (non-PGM). Currently, the most promising non-PGM systems are comprised of transition metal-nitrogen-carbon (M-N-C) containing catalysts. Despite several decades of effort to obtain the "perfect" M-N-C catalyst, there is still a fair amount of work to be done mainly towards understanding the origin of ORR activity in these complex M-N-C systems. The objective in these studies is to design the optimal active structure that is able to provide high and selective performance sustained even in very corrosive environments. Element-specific in-situ X-ray absorption spectroscopy (XAS) coupled with standard electrochemical methodology (mainly Rotating Ring Disc Eelectrode, RRDE) is a great tool to study surface active catalytic systems. With a careful experimental design, in-situ XAS is able to provide very useful mechanistic information regarding structural properties of the active centers and their behavior in simulated electrochemical environments.;Chapter 1 contains a brief description of fundamental aspects of the oxygen reduction reaction, and related challenges. This includes: electrolyte-dependent general description of the ORR mechanistic pathways, and currently known relations between electronic/structural properties of known PGM and non-PGM materials and their catalytic activity. The major electroanalytical and spectroscopic techniques are also discussed, aiming to provide introductory information to the reader needed to understand the experimental work discussed in the following chapters. As the main point of interest is ORR kinetics, which comprise the performance and degradation modes in an aqueous environment, Chapter 2 discusses comparative characteristics of mechanistic ORR pathways (in acid and alkaline media) with a group of the M-N-C catalysts synthesized via various routes. The electroanalytical studies shown in Chapter 2 are followed by more detailed mechanistic investigations (in Chapter 3) wherein the ORR kinetics on the M-N-C catalysts is investigated using in-situ spectro-electrochemical XAS methodologies of transition metal centers. Different forms of the metals and their mechanistic roles are investigated by ORR kinetic studies and behavioral monitoring after selective removal or blocking each of the moieties. The information obtained by the mechanistic studies are used in Chapter 4 to discuss the effect of chloride anions on the overall M-N-C activity with the aim to predict their potential use as O2-consuming cathodes in industrial environments involving presence of the chloride species, known to be a strong poison for platinum-based catalysts. Finally, Chapter 5 shows performance non-PGM catalysts developed at NEU based on carbon supported polymer and self-supported Metal Organic Framework (MOF) iron comprising M-N-C catalysts as oxygen depolarized cathodes for recycling of chlorine gas from hydrochloric acid, a common bi-product in industrial chemical plants. Chapter 5 discusses structure-property relationship of the M-N-C catalysts, and their iron-based active centers to overall catalytic performance and stability in such corrosive environment as concentrated hydrochloric acid. The Chapter 5 also covers a promising preliminary study of utilization of the M-N-C catalysts as Oxygen De-polarized Cathodes (ODC) in the chlor-alkali process for Cl2-production. Finally, Chapter 6 summarized the work presented here and discusses future perspectives for applications of the non-PGM catalysts.