Understanding non-precious metal catalysts for the oxygen reduction reaction and investigating corrosion and surface properties of carbon-doped aluminum alloys

The development of sustainable technologies to address the increasing global energy demand, to enable new ways to conserve energy, and to reduce the environmental impact of energy production and consumption is highly desirable. In many cases, a fundamental understanding is crucial for the discovery and improvement of such technologies. In particular, fundamental studies involving electrocatalysis and corrosion can offer promising solutions to some of the most difficult challenges. The first chapter introduces each of these strategies in more detail. The next three chapters of this thesis describe efforts undertaken to understand the active species within non-precious metal catalysts for potential future applications in fuel cells as a replacement for the more costly Pt-based catalysts used today. The final chapter reports on the electrochemical properties of a new type of Al alloy into which carbon has been added via a new processing method. Together, these studies provide important direction for further research and development of sustainable technologies. Due to the decreasing supply and environmental impact of fossil fuels as a source of energy, the development and utilization of fuel cells as energy conversion devices has increased recently due to their higher theoretical efficiency and lack of harmful byproducts. An additional benefit of fuel cells is that the fuel itself can be generated from renewable energy sources such as wind and solar and stored or transported wherever and whenever it is needed. Despite these desirable features, the efficiency of a fuel cell remains relatively low due to the slow kinetics of the oxygen reduction reaction (ORR). Currently, Pt-based catalysts are used to facilitate the ORR in fuel cells but they are expensive and are still far from reaching the maximum theoretical efficiency. As a result, significant effort has been given to develop non-precious metal (NPM) catalysts over the past several decades. Chapter 2 and Chapter 3 describe new methods to clarify the identity of the active species in Fe-based NPM ORR catalysts. In my work gas-phase Cl2 and H2 treatments are used to alter the Fe species in a NPM catalyst and solution-based treatments with H2SO4 and H2O2 are used to remove metal species from the material. Through the use of electrochemical methods, the activity and selectivity of these catalysts before and after treatment is determined and compared. Additionally, surface and bulk characterization using several different methods reveals the catalyst’s structure and the chemical species present. These studies show that metallic Fe nanoparticles encapsulated in N-doped carbon are the locus of ORR activity in NPM catalysts. Chapter 4 describes a mechanistic study utilizing the kinetic isotope effect (KIE) to compare the ORR pathway on NPM catalysts with precious metal catalysts Pt and Pd. The use of KIE in this case is able to provide information on the role of protons (H+) in the ORR mechanism. This study indicates that there is a KIE observed for a NPM catalyst but a KIE is not observed for Pt or Pd. These findings suggest that NPM catalysts operate via a different pathway than precious metal ones, providing guidance for future catalyst design. Another method to reduce the use of fossil fuels for transportation is the utilization of Al as a light-weight building material. Currently, Al is protected from corrosion using chromate conversion coating in which the Al parts are treated in chromic acid solutions to create a passivation layer. However, chromic acid is highly toxic and alternative strategies for corrosion protection are needed. One potential solution is the formation of “covetic” Al alloys by adding carbon to molten Al and applying a high voltage and current to the mixture. Previous work has demonstrated an improvement in the mechanical properties of covetics prepared via this process. Chapter 5 describes an electrochemical investigation of Al covetics relating to their corrosion and surface properties. Electrochemical methods are used to understand the corrosion potential and rate of corrosion on several materials and surface characterization is performed to interrogate the surface properties. These studies reveal that the corrosion potential of the covetic is shifted to higher potential, which is explained by significant differences in the surface structure of the covetic as compared to the parent material. Specifically, changes in grain size and chemical species on the surface of the covetic material contribute to its corrosion behavior

Microfluidic fabrication of water-in-water (w/w) jets and emulsions

We demonstrate the generation of water-in-water (w/w) jets and emulsions by combining droplet microfluidics and aqueous two-phase systems (ATPS). The application of ATPS in microfluidics has been hampered by the low interfacial tension between typical aqueous phases. The low tension makes it difficult to form w/w droplets with conventional droplet microfluidic approaches. We show that by mechanically perturbing a stable w/w jet, w/w emulsions can be prepared in a controlled and reproducible fashion. We also characterize the encapsulation ability of w/w emulsions and demonstrate that their encapsulation efficiency can be significantly enhanced by inducing formation of precipitates and gels at the w/w interfaces. Our work suggests a biologically and environmentally friendly platform for droplet microfluidics and establishes the potential of w/w droplet microfluidics for encapsulation-related applications

Observation of an Inverse Kinetic Isotope Effect in Oxygen Evolution Electrochemistry

Earth-abundant and inexpensive catalysts with low overpotential and high durability are central to the development of efficient water-splitting electrolyzers. However, improvements in catalyst design and preparation are currently hampered by the lack of a detailed understanding of the reaction mechanisms of the oxygen evolution reaction (OER) facilitated by nonprecious-metal (NPM) catalysts. In this paper, we conducted a kinetic isotope effect (KIE) study in an effort to identify the rate-determining step (RDS) of these intricate electrocatalytic reactions involving multiple proton-coupled electron transfer (PCET) processes. We observed an inverse KIE for OER catalyzed by Ni and Co electrodes. These results contribute to a more complete understanding of the OER mechanism and allow for the future development of improved NPM catalysts

Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect

The development of non-precious-metal (NPM) catalysts to replace the Pt alloys currently used in fuel cells to facilitate the oxygen reduction reaction (ORR) is a vital step in the widespread utilization of fuel cells. Currently, the ORR mechanism for NPM catalysts is not well understood, prohibiting the design and preparation of improved NPM catalysts. We conducted a kinetic isotope effect (KIE) study to identify the rate-determining step (RDS) of this intricate electrocatalytic reaction involving multiple proton-coupled electron transfer (PCET) processes. We observed a KIE of about 2 for the ORR catalyzed by a NPM catalyst, which demonstrates that for these electrocatalysts protons are involved in the RDS during ORR. These results contribute to a more complete understanding of the ORR mechanism and suggest that the design of future NPM catalysts must include careful consideration of the role of protons during ORR

Revealing the Role of the Metal in Non-Precious-Metal Catalysts for Oxygen Reduction via Selective Removal of Fe

Non-precious-metal catalysts have been investigated as alternatives to Pt-based oxygen reduction reaction catalysts for more than 50 years. While the incorporation of a metal is known to be necessary to generate a catalyst with high activity, the exact role of the metal is still not well-understood. In this work, we prepare an active oxygen reduction reaction catalyst containing Fe and then selectively remove the Fe from the catalyst while preserving the carbon and nitrogen species. By comparing the oxygen reduction reaction activity of the catalyst before and after treatment, we show that in the absence of Fe the carbon and nitrogen sites in the catalyst exhibit a larger overpotential and lower selectivity for the 4<i>e</i>^– reduction of oxygen in both acidic and alkaline conditions. These findings reveal the direct involvement of the metal in the active site of non-precious-metal catalysts and provide important guidance for future catalyst improvements