Catalytic Properties Of Cobalt, Nickel, And Copper Complexes Containing Pyridine-Amine Pentadentate Ligands For Electrochemical Production Of Hydrogen From Aqueous Media

Rapid increase of global energy demands and CO2-overdischarge have prompted an urgent search for ulilization of new renewable energy, such as wind and solar energy. But the uneven distribution in seasons and districts of these energies requires new technological advances to overcome the challenges of on-demand energy serving and transport. Producing hydrogen by splitting water with renewable electricity is considered as an ideal way for developing sustainable energy systems. Platinum is the lowest power loss electrochemical hydrogen generation catalyst with the highest catalytic activity in acidic aqueous solutions. However, Pt is one of the scarcest elements on earth and is very expensive. Commercial hydrogen electrolyzers hanker after non-noble metal catalysts to break the choke point of Pt. Development of electrocatalysts that are made from earth-abundant elements and work in fully aqueous solutions with high activity, low overpotential, and good durability is one of the crucial issues to be solved for the realization of an energy-efficient and cost-effective transformation of electric power to hydrogen.In chapter 2, four new cobalt complexes [(Ll)CoNCCH3]2+(Al) (L1= N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine), [(Ll)CoOH2]2+(A2), [(L2)CoNCCH3]2+(Bl) (L2=N-benzyl-N,N',N-tris(2-pyridylmethyl) propylenediamine) and [(L2)CoOH2]2+(B2) containing a tripyridine-diamine N5-pentadentate ligand were prepared and structurally identified. Electrochemical studies using differential pulse polarography and cyclic voltammetry (CV) show that a small variation in the back linkage of the tripyridine-diamine ligand, from ethanediamine to propanediamine, has a considerable influence on the electrochemical properties of such cobalt complexes. Complex A2 with ethanediamine linkage, was found to be a highly active catalyst for electrochemical H2 production from neutral water, with a turnover frequency (TOF) of 860 mol H2 (mol cat)-1 h-1 (cm2 Hg)-1 over 60 h controlled potential electrolysis (CPE) experiment at-1.25 V vs. SHE in pH 7 phosphate buffer solution, without considerable deactivation.In chapter 3, mononuclear nickel complexes [(L3)NiR] 2+(L3= 1,1-di(2-pyridinyl)-N,N-bis (2-pyridinylmethyl)methanamine, C1, R= CH3CN and C2, R= H2O), [(Ll)NiR] 2+(D1, R= CH3CN and D2, R=H2O) and [(L4)NiL] 2+(L4= N,N'-dibenzyl-N-(2-(benzyl(2-pyridinylmethyl)amino)ethyl)-N'-(2-pyridinylmethyl)-ethane-1, 2-diamine,.E1, R=CH3CN and E2, R=H2O) with a series of amine-pyridine-based N5-pentadentate ligands, L1, L3, and L4, were prepared and structurally characterized. The electrochemical studies of C1-E1 in THF show that the first reduction potential for the Ni?/Ni? couple apparently shifts to positive direction with the increasment of amine-N atoms in an amine-pyridine-based N5-pentadentate ligand, while the potential of the second one is less affected. All three nickel complexes can electrochemically catalyze water reduction to produce H2 in neutral aqueous solutions but with significantly different catalytic activities. Among these nickel complexes, D2 is the most active and robust electrocatalyst for hydrogen generation from neutral aqueous solutions. The H2 evolving TOF of D2 in pH 7 phosphate buffer solution reaches 1650 mol H2 (mol cat)-1 h-1 (cm2 Hg)-1 at-1.25 V vs. SHE, which is more than one-fold higher than that displayed by the analogous cobalt catalyst at the same applied potential and 3 to 37-fold higher than the activities reported for other nonprecious metal-based molecular catalysts of Nx-multidentate ligands at applied potentials of-1.30 to-1.40 V under similar measuring conditions.In chapter 4, an ionic copper complex [(L1)Cu]2+(G), which contains an amine-pyridine-based pentadentate ligand with five coordinating nitrogen atoms, displayed a hydrogen generation rate constant (kobs) of over 10000 s-1 with a 420 mV onset overpotential in buffer solution at pH 2.5. The CPE of [(L1)Cu]2+ in pH 2.5 phosphate buffer at-0.90 V vs. SHE over two hours using a glassy carbon electrode gave a TON of 1.4×104 mol H2 (mol cat)-1 cm-2 with a Faradaic efficiency of approximately 96%, which corresponds to a TOF of approximately 2.0 mol H2(mol cat)-1 s-1cm-2. The results obtained from electrochemical and spectroscopic studies reveal that the H2 generation reaction takes place, by two successive protoncoupled reduction processes, with protonation occurring at the ligand in the first reduction step and at the Cu? center in the second step to afford a [(L1H)Cu?(H)]2+ copper hydride species, which could release H2 and regenerate the Cu? catalyst in its initial form.The Cu-CuxO-Pt-1 catalyst was conveniently prepared by electrodeposition of cheap and simple copper(?) ionic complexes (G) with a Pt foil as counter electrode in aqueous solution. This Pt-doped copper nanoparticulate catalyst (Cu-CuxO-Pt-1), with a small Tafel slope (44 mV dec-1) and a large exchange current density (j0=1.601 mA cm-2), displayed an apparently lower onset overpotential (?onset< 10 mV) and significantly higher activity (20 mA cm-2 at ?=-45 mV and 500 mA cm-2 at ?=-370 mV) than a Pt foil or Pt/C catalyst in neutral buffer solution. More impressively, the catalytic activity of the Cu-CuxO-Pt-1 film in a neutral solution is compatible to a Pt electrode in 0.5 M H2SO4 solution and the mass activity of Pt in the Cu-CuxO-Pt-1 film for electrolysis of neutral water is 11-fold higher than that of the CuPt electrode in strongly acidic solution. The Cu-CuO-Pt-1 film maintained its catalytic activity for more than 100 h in the bulk electrolysis of a neutral solution at ?=-200 mV (?190mAcm-2).