Transition Metal Complex Modified Eiectrodes Towards Electro-and Photocatalytic Water Oxidation

One of the most promising techniques for future energy production is artificial photosynthesis that directly converts solar energy, the origin of nearly all energy forms on the planet, into chemical energy by splitting water into O2and H2. Water oxidation is considered as the bottleneck of artificial photosynthesis because it could generate electrons and protons necessary for hydrogen production but it is an uphill energy reaction involving four electrons and four protons (2H2Oâ†'O2+4H++4e-). To efficiently convert solar energy, robust water oxidation catalysts (WOCs) and WOC modified electrodes are desired.Rul has been known to be an efficient molecular WOC. In order to carry out electrochemical water oxidation, pyrene functionalized Rul was successfully immobilized on an ITO electrode by noncovalent Ï€-Ï€ stacking interactions. Herein, MWCNTs serve as an ideal platform for catalyst loading and a medium for fast electron transfer. The overpotentials for water oxidation with Rul/MWCNTs/ITO anode were found to be as low as280mV. Sustained current density of220mA/cm2was obtained at a relatively low applied potential of1.4V vs NHE in nonbuffered neutral aqueous solution. The long-term electrolysis over10h gave a total TON of11000cycles accounting for an average TOF of about0.3s-1. The highly convenient Ï€-Ï€ stacking approach for functionalization of the electrode and the high tunability of the molecular catalyst are of considerable importance for the development of optimized oxygen-evolving anodes.Cobalt-oxo cubane clusters were immobilized on a Nafion coated FTO electrode and an α-Fe2O3photoanode as surface catalysts for water oxidation. The performance of electrochemical water splitting indicated that these earth-abundant metal complexes retain their homogeneous reactivity on both electrodes. For FTO/Nafion/Col, a stable current density of230μA/cm2persists for10h in neutral phosphate buffer under an applied potential of1.2V. For FTO/Nafion/α-Fe2O3/Col, a steady photocurrent is maintained above350μA/cm2under extended periods of illumination at0.6V. This is the first time for water oxidation to be achieved by combination of an inorganic semiconductor and a noble metal-free molecular catalyst.We also developed an active inorganic metal oxide WOCs because of its high stability for electrocatalytic oxygen evolution. Two amorphous cobalt oxide films (Co-W film and Co-Mo film) with high activity for electrocatalytic water oxidation were prepared by fast and simple electrodeposition from Na2WO4and Na2MoO4aqueous solutions containing Co2+. In Na2WO4and Na2MoO4solutions (pH8), sustained anodic current densities up to1.45mA/cm2and0.95mA/cm2were obtained for Co-W film and Co-Mo film on FTO substrates at1.4V, respectively. Significantly, in the long-term electrolyses for13h, Co-W film exhibited improved stability in cobalt-free buffer solution in comparison with the early reported Co-Pi thin film.The activity of eleven separated iron complexes with various coordination environments and nine in situ-generated iron complexes towards catalytic water oxidation were examined in aqueous solutions with Ce(NH4)2(NO3)6as the oxidant. Two iron complexes Fe3and Fe6bearing tridentate and tetradentate macrocyclic ligands were found to be novel WOCs. The one with tetradentate ligand exhibited a promising activity with a turnover number of65for oxygen evolution.In the thesis, water oxidation electrodes and one water oxidation photoanode modified by noble-metal-free molecular catalyst were developed for water splitting device. So, the above results are significative to build applicable solar water splitting system.