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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2024)

Since the 1990s, the advent of lithium-ion batteries has revolutionized energy storage in compact, high-density applications ranging from personal devices to passenger automobiles.

Over the past 5 years, they have become crucial to making electrical grids resilient to weather anomalies. In this context, we will discuss nascent chemistry and system design around two energy storage/conversion systems: direct ammonia fuel cells (DAFCs), and membrane-less redox-flow batteries (RFBs). Both systems share a common layout and utilize the membrane-electrode assembly (MEA), which sandwiches the anode and cathode electrodes with aselective ion-exchange membrane.

DAFCs operate as an open system, where ammonia and oxygen streams are converted into dinitrogen and water within the MEA while generating electricity for external loads. While ammonia offers logistical advantages over hydrogen, it presents a challenge in terms of the scarcity of efficient electrocatalysts for the sluggish 6e- NH3 oxidation. Our work emerges from the fundamental necessity for developing these electrocatalysts for protic conditions. We have found a molecular Ru complex that catalyzes rapid NH3 oxidation in aqueous conditions, demonstrating high selectivity for N2 – hitherto unprecedented among molecular electrocatalysts under aqueous conditions.

Shifting focus to the membrane component, we will describe our efforts to improve membrane-less RFBs. RFBs employ the MEAs in a closed system, with anolyte and catholyte solutions containing redox active compounds circulating across the anode and cathode during operation. The ion-selective membrane in typical RFBs prevents self-discharge by separating the anolyte and catholyte, constituting a significant portion of the MEA cost. We will explore the shortcomings on previous attempts at biphasic membrane-less RFBs and explain a straightforward modification to the cell layout that overcomes interfacial self-discharge in these biphasic cells, elevating their Coulombic efficiency to >99%. In scaling up these cells,we will peek at ion transfer kinetics across liquid-liquid interfaces.

    Cover page: Forays in electrochemistry: NH3-to-N2 oxidation electrocatalysis and membrane-less redoxflow batteries
    • Article
    • Peer Reviewed
    UC Santa Barbara Previously Published Works (2023)
    Reaction of MBr2 with 3 equiv of [K(18-crown-6)][O2N2CPh3] generates the trityl diazeniumdiolate complexes [K(18-crown-6)][M(O2N2CPh3)3] (M = Co, 2; Fe, 3) in good yields. Irradiation of 2 and 3 using 371 nm light led to NO formation in 10 and 1% yields (calculated assuming a maximum of 6 equiv of NO produced per complex), respectively. N2O was also formed during the photolysis of 2, in 63% yield, whereas photolysis of 3 led to the formation of N2O, as well as Ph3CN(H)OCPh3, in 37 and 5% yields, respectively. These products are indicative of diazeniumdiolate fragmentation via both C-N and N-N bond cleavage pathways. In contrast, oxidation of complexes 2 and 3 with 1.2 equiv of [Ag(MeCN)4][PF6] led to N2O formation but no NO formation, suggesting that diazeniumdiolate fragmentation occurs exclusively via C-N bond cleavage under these conditions. While the photolytic yields of NO are modest, they represent a 10- to 100-fold increase compared to the previously reported Zn congener, suggesting that the presence of a redox-active metal center favors NO formation upon trityl diazeniumdiolate fragmentation.
      Cover page: NO and N2O Release from the Trityl Diazeniumdiolate Complexes [M(O2N2CPh3)3]− (M = Fe, Co)Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2021)
      We report the facile activation of aryl E-H (ArEH; E = N, O, S; Ar = Ph or C6F5) or ammonia N-H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1-). Substantial decreases in E-H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol-1 in H2N-H BDFE upon coordination was experimentally determined.
        Cover page: Facile proton-coupled electron transfer enabled by coordination-induced E–H bond weakening to boron
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