Author: Michael Kenneth Bates

Publisher:

Published: 2015

Total Pages: 162

ISBN-13:

Understanding the fundamentals of electrochemical interfaces will undoubtedly reveal a path forward towards a society based on clean and renewable energy. In particular, it has been proposed that hydrogen can play a major role as an energy carrier of the future. To fully utilize the clean energy potential of a hydrogen economy, it is vital to produce hydrogen via water electrolysis, thus avoiding co-production of CO2 inherent to reformate hydrogen. While significant research efforts elsewhere are focused on photo-chemical hydrogen production from water, the inherent low efficiency of this method would require a massive land-use footprint to achieve sufficient hydrogen production rates to integrate hydrogen into energy markets. Thus, this research has primarily focused on the water splitting reactions on base-metal catalysts in the alkaline environment. Development of high-performance base-metal catalysts will help move alkaline water electrolysis to the forefront of hydrogen production methods, and when paired with solar and wind energy production, represents a clean and renewable energy economy. In addition to the water electrolysis reactions, research was conducted to understand the de-activation of reversible hydrogen electrodes in the corrosive environment of the hydrogen-bromine redox flow battery. Redox flow batteries represent a promising energy storage option to overcome the intermittency challenge of wind and solar energy production methods. Optimization of modular and scalable energy storage technology will allow higher penetration of renewable wind and solar energy into the grid. In Chapter 1, an overview of renewable energy production methods and energy storage options is presented. In addition, the fundamentals of electrochemical analysis and physical characterization of the catalysts are discussed. Chapter 2 reports the development of a Ni-Cr/C electrocatalyst with unprecedented mass-activity for the hydrogen evolution reaction (HER) in alkaline electrolyte. The HER kinetics of numerous binary & ternary Ni-alloys and composite Ni/metal-oxide/C samples were evaluated in aqueous 0.1 M KOH electrolyte. The highest HER mass-activity was observed for Ni-Cr materials which exhibit metallic Ni as well as NiOx and Cr2O3 phases as determined by ex-situ XRD and in-situ XAS analysis. The on-set of the HER is significantly improved compared to numerous binary and ternary Ni-alloys - including state-of-the-art Ni-Mo materials. It is likely that at adjacent Ni/NiOx sites, the oxide site facilitates formation of adsorbed hydroxide (OHads) from the reactant (H2O) thus minimizing the high activation energy of cleaving the H-OH bond to form the Hads HER intermediate on the metallic Ni site. This is confirmed by in-situ XAS studies which show that the synergistic HER enhancement is due to NiOx content and that the Cr2O3 appears to stabilize the composite NiOx component under HER conditions (where NiOx would typically be reduced to metallic Ni0) Furthermore in contrast to Pt, the Ni(Ox)/Cr2O3 catalyst appears resistant to poisoning by the anion exchange ionomer (AEI), a serious consideration when applied to an anionic polymer electrolyte interface. Furthermore a model of the double layer interface is proposed, which helps explain the observed ensemble effect in the presence of AEI. In Chapter 3, Ni-Fe and Ni-Fe-Co mixed-metal-oxide (MMO) films were investigated for oxygen evolution reaction (OER) activity in 0.1M KOH on high surface area Raney-Nickel supports. During investigations of MMO activity, aniline was identified as a useful "capping agent" for synthesis of high-surface area MMO-polyaniline (PANI) composite materials. A Ni-Fe-Co/PANI-Raney-Ni catalyst was developed which exhibits enhanced mass-activity compared to state-of-the-art Ni-Fe OER electrocatalysts reported to date. Furthermore, in-situ XAS analysis revealed charge-transfer effects of MMOs in which the average oxidation state of the OER-active NiOx(OH)y sites is affected by the binary or ternary components (Fe &/or Co). Cyclic voltammetry results show changes in the potential of the Ni2+/3+ transitions in the presence of binary or ternary metals. In-situ XAS analysis confirms that the redox peaks can be attributed to the Ni sites and the shifts in the XANES peak as a function of applied potential indicates that Fe acts to stabilize Ni in the 2+ oxidation state, while Co facilitates oxidation to the 3+ state. The enhanced OER activity of the ternary Ni-Fe-Co/PANI-Raney catalyst is likely due to "activation" of the conductive Ni(III)OOH phase at lower overpotential due to the charge-transfer effects of the cobalt component. The morphology of the MMO catalyst film on PANI/Raney-Ni support provides excellent dispersion of active-sites and should maintain high active-site utilization for catalyst loading on gas-diffusion electrodes. In Chapter 4, the de-activation of reversible-hydrogen electrode catalysts was investigated and the development of a Pt-Ir-Nx/C catalyst is reported, which exhibits significantly increased stability in the HBr/Br2 electrolyte. Initial screening of Rh- and Ru-chalcogenides (oxides, sulfides and selenides) indicates that these non-Pt catalysts do not exhibit sufficient hydrogen reaction kinetics for use in the hydrogen electrode of a H2-Br2 redox flow battery (RFB). However, a standard Pt/C catalyst suffered from rapid and irreversible de-activation upon high-voltage cycling or exposure to Br2. In contrast a Pt-Ir/C catalyst exhibited increased tolerance to high-voltage cycling and in particular showed recovery of electrocatalytic activity after reversible de-activation (presumably from bromide adsorption and subsequent oxidative bromide stripping). Under the harshest testing conditions of high-voltage cycling or exposure to Br2 the Pt-based catalyst showed a trend in stability: Pt