Electrocatalysts Based On Organic Macrocycles For Electrochemical Water Splitting

Water splitting, which is the dissociation of water into hydrogen and oxygen gases, can be separated into two half reactions corresponding to the hydrogen evolution reaction and oxygen evolution reaction (HER and OER, respectively). Both reactions offer a promising way for storing energy into the form of chemical bonds, which is an important strategy for the development of clean-energy technologies. A main area of interest while studying HER and OER is to design effective and robust electrocatalysts made from earth-abundant materials. This dissertation describes several homogeneous and heterogeneous systems for the electrocatalytic generation of hydrogen and oxygen gases. In Chapter 2, we present a metal-free (free-base) perfluorinated porphyrin as an electrocatalyst for HER, namely meso-tetra(pentafluorophenyl)porphyrin (1). Compound 1 is electrocatalytically active for hydrogen gas generation in the presence of p-toluenesulfonic acid. The electrochemical potential of hydrogen evolution (–1.31 V vs Fc/Fc+ in THF) is comparable to those metal containing electrocatalysts such as metallated porphyrins or other metallated macrocycles. In combination of experimental data and DFT computations, we propose the most favorable hydrogen generation mechanism to be a (1) reduction, (2) protonation, (3) reduction, (4) protonation (E-P-E-P) pathway. Kinetic studies from foot-of-the-wave analysis (FOWA) show a linear relationship between the observed rate constants (s) and acid concentrations. Based upon these results, we have pursued several porphyrin-based polymeric systems as heterogeneous electrocatalysts, which are described in Chapters 3 and 4. We have synthesized a crystalline CoTcPP-based [TcPP = the dianion of meso-tetra(4-carboxyphenyl)porphyrin] polymeric system, 2, as a HER electrocatalyst in acidic aqueous media. Polymer 2 shows a surface area of 441.74 m2/g, while the discrete CoTcPP molecule (3) has a surface area of 3.44 m2/g. The HER catalytic performance of 2 and 3 was evaluated by means of linear sweep voltammetry in the presence of 0.5 M H2SO4 aqueous solution. To achieve 10 mA/cm2 cathodic current density, 2 and 3 respectively require an overpotential of 0.475 V and 0.666 V, providing strong evidence that the extended network of cobalt porphyrin leads to enhanced HER efficiency. The polymer also shows great tolerance for HER electrolysis in the presence of acid remaining active over 10 h. Similarly, we constructed several amorphous cobalt porphyrin and iron porphyrin based organic polymers as electrocatalysts to heterogeneously generate oxygen gas in basic aqueous solution (0.1 M KOH). The porphyrin organic polymer [(Por)OP] and its fluorinated version [F(Por)OP] were synthesized from an one-step condensation reaction without further cross-coupling reactions, followed by direct metallation to form the metalloporphyrin polymers Co(Por)OP, CoF(Por)OP, Fe(Por)OP and FeF(Por)OP using Co2+ and Fe2+ accordingly. All metalloporphyrin polymers are active for the electrocatalytic oxygen evolution with modest overpotentials. The cobalt fluorinated porphyrin polymer, [CoF(Por)OP], shows the best catalytic efficiency (η = 0.456 at 10 mA/cm2), while both the metal-free polymers, Por(OP) and FPor(OP), show negligible catalytic activity. In Chapter 5, we extend the study of the heterogenization of porphyrin-based electrocatalysts by the immobilization of metalloporphyrin molecules using a supporting platform. We report the use of zirconium phosphate (ZrP) layered nanomaterials as a catalyst support for the intercalation of molecular electrocatalysts. Two cobalt porphyrins, namely CoTsPP [TsPP = the dianion of meso-tetra(4-sulfonatophenyl)porphyrin] and CoTcPP [TcPP = the dianion of meso-tetra(4-carboxyphenyl)porphyrin], were intercalated into ZrP layers and evaluated as heterogeneous OER electrocatalysts. Standard spectroscopic techniques including Fourier-transform infrared (FT-IR) spectroscopy, x-ray powder diffraction (XRPD) measurements, elemental mapping, energy dispersive x-ray (EDX) analysis and x-ray photoelectron spectroscopy (XPS) were utilized to determine the successful intercalation of cobalt porphyrin molecules into ZrP. The OER electrocatalytic performance of both intercalated species and the pristine α-ZrP are assessed by means of cyclic voltammetry in 0.1 M KOH aqueous solution. To reach 10 mA/cm2 current density, CoTsPP/ZrP and CoTcPP/ZrP require an overpotential of 0.462 and 0.467 V, respectively. To compare, α-ZrP shows negligible electrocatalytic activity for OER. Another molecular HER electrocatalyst is discussed in Chapter 6, Co(DippF)2 (where DippF is the anion of N,N’-bis[2,6-diisopropylphenyl]-formamidine), (4), which is able to electrochemically produce hydrogen gas from the reduction of organic acids in homogeneous solutions. Compound 4 has a distorted square planar structure as evidenced through single crystal x-ray crystallography, and an effective magnetic moment of 4.13 that corresponds to three unpaired electrons. Catalyst 4 shows an irreversible cathodic peak at –1.59 V vs Fc/Fc+ which is assigned to the reduction of CoII to CoI. In the presence of organic acids the onset of catalytic current is observed at –1.2 V, –1.45 V and –1.89 V vs. Fc/Fc+ with p-toluenesulfonic acid, benzoic acid and phenol as the proton sources, respectively, in MeCN as the solvent. Detection of hydrogen gas was obtained by GC-MS with Faradaic efficiencies ranging from 85% to 100%. Kinetic studies using foot-of-the-wave analysis (FOWA) reveal a linear dependence of the observed rate constant against acid concentration in the range of 0.065 to 10.02 s-1. Chapter 7 describes several cobalt molybdenum disulfide (CoMoS2) catalysts which are active electrocatalysts for the production of hydrogen gas. These highly-active catalysts are obtained from pretreatment of ammonium tetrathiomolybdate (ATM) with different amines precursors. Electrochemical studies indicate that these CoMoS2 materials exhibit improved catalytic performance for hydrogen gas production with overpotentials ranging from 0.127 to 0.144 V, which are significantly less than CoMoS2 synthesized directly from ATM under the same synthetic techniques (0.173 V). These CoMoS2 catalysts are also stable in the presence of strong acidic media after 10 h while maintaining their efficiencies for hydrogen gas evolution