Mechanistic Studies And Applications Of Oxygen Activation For Energy Conversion And Storage

Author : Yu-Heng Wang (Ph.D.)
Publisher :
ISBN 13 :
Total Pages : 407 pages
Book Rating : 4.:/5 (131 download)

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Download or read book Homogeneous Catalytic Oxygen Reduction by Monomeric Cobalt Complexes written by Yu-Heng Wang (Ph.D.) and published by . This book was released on 2018 with total page 407 pages. Available in PDF, EPUB and Kindle. Book excerpt: The oxygen reduction reaction (ORR) plays an important role in various life processes such as respiration and in energy conversion systems such as fuel cells and metal-air batteries. Achieving a selective and efficient ORR remains a significant challenge in energy conversion, where the sluggish kinetics of the ORR has also restricted its practical application. Platinum group metal (PGM) catalysts are currently the best-performing candidates for efficient reduction of O2 with overpotential less than 300 mV. However, scientists are exploring non-PGM ORR catalysts, due to their high abundance and low cost. Molecular first row transition-metal complexes, such as those macrocyclic ligands containing cobalt are the main representatives of non-precious ORR catalysts. This thesis details the comprehensive investigation of the selectivity, overpotential, mechanistic studies, and linear free energy relationship (LFER) analysis of homogeneous O2 reduction to H2O2 and H2O catalyzed by a series of monomeric cobalt complexes. Chapters 2-4 of this thesis depicts the homogeneous 2e[-]/2H+ O2 reduction catalyzed by a series of cobalt complexes bearing tetradentate N2O2-based ligands (Co(N2O2)). These studies show that H2O2 is directly produced from molecular O2 with an effective overpotential as low as 90 mV. A linear dependence of logarithm of turnover frequency (log(TOF)) is observed with respect to effective overpotential suggesting there exists a trade-off between the rate and overpotential for ORR with these Co(N2O2) complexes. However, the dependence is weaker compared with that of iron porphyrin (Fe(por)) complexes, which is rationalized by the different influence of effective overpotential on their turnover-limiting step. The subsequent mechanistic studies reveal that the protonation on the proximal oxygen atom of the CoIII(OOH) intermediate is the rate-limiting step in the ORR. Density functional theory (DFT) studies, in combination with kinetic and electrochemical studies, provide deeper mechanistic insights into the O2 reduction catalyzed by Co(N2O2) complexes. The small Brønsted coefficient and O2-independent rate law imply that the low-overpotential O2 reduction is achievable because the ORR rate is relatively insusceptible to the effective overpotential. The performance of monomeric cobalt ORR catalysts bearing N4-macrocyclic ligands reported in the literature is evaluated on the basis of their TOF and effective overpotential. It is shown for the first time that they all fall in the linear relationships between log(TOF) and effective overpotential. Chapters 5-7 of this thesis highlight the exploration of the homogeneous 4e[-]/4H+ reduction of O2 to H2O catalyzed by a series of cobalt porphyrin (Co(por)) complexes. The catalysis-initiating potential of the Co(por) complexes (half-wave potential, E1/2(CoIII/II)) is found to be independent of the pKa values of the acids, but the potential shift for O2/H2O redox couples is nearly 59 mV/pKa in organic media. This outcome suggests that the effective overpotential is correlated with the acid strength following Nernstian behavior of 59 mV/pKa unit. Thus, selective homogeneous 4e[-]/4H+ O2 reduction to H2O with effective overpotentials as low as 50 mV is achieved by using high-potential Co(por) complexes and modulating the O2/H2O redox couple with different acids. A second effect of this modulation is that the selective reduction of O2 to H2O is observed when the E1/2(CoIII/II) of the monomeric cobalt complexes are above the O2/H2O2 redox couple, implying that the ORR selectivity is tunable based on thermodynamic constraints. A crossover in the LFER of the O2 reduction mediated by the same Co(por) complex exhibits that the susceptibility of the log(TOF) to effective overpotential changes with the ORR selectivity (H2O vs. H2O2). The different slopes of the relationship between log(TOF) and effective overpotential for O2/H2O and O2/H2O2 is elucidated by their distinctive rate laws and catalytic mechanisms. Finally, the different LFERs for the O2 reduction catalyzed by monomeric cobalt and iron complexes between the log(TOF) and effective overpotential are attributed to their different Brønsted coefficients, rate laws, and catalytic mechanisms, and the performance of molecular cobalt and iron ORR catalysts are quantitatively evaluated with LFER analyses. Collectively, this thesis delineates the opportunities for the utilization of molecular cobalt catalysts for achieving selective 2e-/2H+ and 4e[-]/4H+ reduction of O2 with low overpotentials. The overpotential can be tuned using different cobalt catalysts and reaction media under homogeneous conditions, and this feature serves as the basis for switching the selectivity from H2O2 to H2O or vice versa based on thermodynamic restraints.