Synergetic molecular approaches towards artificial and photosynthetic water photoelectrolysis

Abstract

Abstract 25 years after the discovery of UV photoelectrolysis of water with TiO 2 by Fujishima and Honda, which triggered high expectations with respect to water splitting using visible light, an evaluation of progress is attempted. On the basis of an analysis of existing electrocatalysts for oxygen evolution alternative reaction possibilities are compared: one-electron transfer, four-electron transfer and the role of intermediates chemically bonded to a catalyst. For the managanese complex of photosynthetic oxygen evolution the puzzling situation is that electrocatalytically poorly active Mn metal centres in a fragile complex are highly catalytic for oxygen evolution. The conclusion is reached that the complex organizes itself into a favourable transition state through an autocatalytic process (export of entropy) that facilities a real (intermediate-free) four-electron transfer. An experimental model system for oscillating hydrogen evolution is discussed to demonstrate the feasibility of synergetic reactions for fuel generation. The energetic and kinetic advantages of a synergetic mechanism of water splitting are outlined and a kinetic model is calculated which simulates oscillations observed in photosynthetic oxygen evolution.