Metalloporphyrins bearing a single proton relay or rhenium-diimines for electrochemical reduction of oxygen and carbon dioxide

Electrochemical reduction of dioxygen (O2) and carbon dioxide (CO2) are two examples of redox reactions that have captured interest as renewable energy solutions. For example, fuel cell technologies demand improvement for reduction of O2 to H2O selectively at low energy costs. The basic requirements for transformations of O2 and CO2 are input of multiple protons (H+) and electrons (e–) (e.g., 4H+/4e– for O2 to H2O). The first half of this thesis focuses on the O2 reduction reaction (ORR) using heterogeneous catalytic platforms. The work starts from Fe-tetra(aryl)porphyrins, which are known ORR catalysts (producing H2O). However, they exhibit high overpotentials and poor stability, making them ill-suited for application. In comparison, heterogeneous Co-derivatives show ORR at lower overpotentials but make mostly H2O2. This is undesirable for fuel cell applications. Following an introduction to proton-coupled redox catalysis (Chapter 1), Chapter 2 presents my work on asymmetric Fe-porphyrin derivatives bearing a single pendant proton relay. Work on heterogeneous ORR shows improvement in catalytic stability on graphite electrodes without compromising selectivity. Chapter 3 shows that Co-derivatives are better catalysts than Fe analogs and use of asymmetric porphyrins shifts selectivity from production of H2O2 to H2O. The electrochemical CO2 reduction reaction (CO2RR) provides a route to turn a greenhouse gas into value-added products. The 2H+/2e– reduction of CO2 to CO is the central attention because CO could be used as a precursor to produce many products, including fuels. Benchmark Fe-porphyrins bearing up to 8 proton relays are known for homogeneous CO2RR. In Chapter 4, I show that an iron porphyrin with one proton relay at a meso-position can achieve a similar benchmark depending on the choice of solvent. Chapter 5 presents that these asymmetric metalloporphyrin derivatives are excellent heterogeneous CO2RR catalysts once immobilized on graphite surfaces, with activation of CO2 near the thermodynamic potential. Another family of CO2RR catalysts, based on ClRe(CO)3 fragments, is described in Chapter 6. Here, I show that Re complexes containing derivatives of 2,2’-pyridylimidazole reduce CO2 at rates that are competitive with state-of-the-art Re derivatives. Immobilization strategies for these Re-complexes onto electrodes are also proposed

Electrocatalytic O2 Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate

The development of electrocatalysts for the selective O2-to-H2O conversion, the O2 reduction reaction (ORR), is of great interest for improving the performance of fuel cells. In this context, molecular catalysts that are known to mediate the 4H+/4e– reduction of O2 to H2O tend to be marred by limited stability and selectivity in controlling the multi-proton and multi-electron transfer steps. Thus, evaluation of new transition metal complexes, including organometallic species, for ORR reactivity could uncover new molecular catalysts with improved properties. We have previously reported the synthesis and characterization of various organometallic PdIII complexes stabilized by the tetradentate ligand N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4). These complexes were shown to react with O2 and undergo oxidatively-induced C–C and C–heteroatom bond formation reactions in the presence of O2. These O2-induced oxidative transformations prompted us to evaluate the ORR reactivity of such organometallic Pd complexes, which to the best of our knowledge has never been studied before for any molecular Pd catalyst. Herein, we report the ORR reactivity of the [(tBuN4)PdIIIMeCl]+ complex, under both homogeneous and heterogeneous conditions in a non-aqueous and acidic aqueous electrolyte, respectively. Cyclic voltammetry and hydrodynamic electrochemical studies for [(tBuN4)PdIIIMeCl]+ revealed the electrocatalytic reduction of O2 to H2O proceeds with Faradaic efficiencies (FE) of 50-70% in the presence of acetic acid (AcOH) in MeCN. The selectivity toward H2O production further improved to a FE of 80-90% in an acidic aqueous medium (pH 0), upon immobilization of the molecular catalyst onto edge plane graphite (EPG) electrodes. Analysis of electrochemical data suggests the formation of a binuclear PdIII intermediate in solution, likely a PdIII-peroxo-PdIII species, which dictates the thermochemistry of the ORR process for [(tBuN4)PdIIIMeCl]+ in MeCN, and thus being a rare example of a bimolecular ORR process. The maximum second-order turnover frequency TOFmax(2) = 2.76 x 108 M–1 sec–1 was determined for 0.32 mM of [(tBuN4)PdIIIMeCl]+ in the presence of 1 M AcOH in O2-saturated MeCN with an overpotential of 0.32 V. By comparison, a comparatively lower TOFmax(2) = 1.25 x 105 M–1 sec–1 at a higher overpotential of 0.8 V was observed for [(tBuN4)PdIIIMeCl]PF6 adsorbed onto EPG electrodes in O2-saturated 1 M H2SO4 aqueous solution. Overall, reported herein is a detailed ORR reactivity study using a novel PdIII organometallic complex and benchmark its selectivity and energetics toward O2 reduction in MeCN and acidic aqueous solutions. </p

A Molecular Cu Electrocatalyst Escalates Ambient Perfluorooctanoic Acid Degradation

Groundwater reservoirs contaminated with per- and polyfluoroalkyl substances (PFASs) need purifying remedies. Perfluorooctanoic acid (PFOA) is the most abundant PFAS in drinking water. Although different degradation strategies for PFOA have been explored, none of them disintegrates the PFOA backbone rapidly under mild conditions. Herein, we report a molecular copper electrocatalyst that assists in the degradation of PFOA up to 93% with 99% defluorination rate within 4 h of cathodic controlled-current electrolysis. The current-normalized pseudo-first-order rate constant has been estimated to be quite high for PFOA decomposition (3.32 L h–1 A–1), indicating its fast degradation at room temperature. Furthermore, comparatively rapid decarboxylation over the first 2 h of electrolysis has been suggested to be the rate-determining step in PFOA degradation. The related Gibbs free energy of activation has been calculated as 22.6 kcal/mol based on the experimental data. In addition, we did not observe the formation of short-alkyl-chain PFASs as byproducts that are typically found in chain-shortening PFAS degradation routes. Instead, free fluoride (F–), trifluoroacetate (CF3COO–), trifluoromethane (CF3H), and tetrafluoromethane (CF4) were detected as fragmented PFOA products along with the evolution of CO2 using gas chromatography (GC), ion chromatography (IC), and gas chromatography-mass spectrometry (GC-MS) techniques, suggesting comprehensive cleavage of C–C bonds in PFOAs. Hence, this study presents an effective method for rapid degradation of PFOA into small ions/molecules

Electrocatalytic H2 Evolution Promoted by a Bioinspired (N2S2)Ni(II) Complex at Low Acid Concentration

We have investigated a bioinspired (N2S2)Ni(II) electrocatalyst that produces H2 from CF3CO2H with a turnover frequency (TOF) of ~200,000 s–1 at low acid concentration (<0.043 M) in MeCN. We also propose an electrochemical mechanism for such an electrocatalyst toward H2 production and benchmarked its activity by comparing its TOF and overpotential with those of other reported molecular Ni H2 evolution electrocatalysts

Characterization of Paramagnetic States in an Organometallic Nickel Hydrogen Evolution Electrocatalyst

Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s-1 and a moderate overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes

Development of high-voltage and high-energy membrane-free nonaqueous lithium-based organic redox flow batteries

Abstract Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm2 and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm2 and at a temperature of 27 °C