The design of bioinspired schemes that couple solar energy conversion to the oxidation of water and the subsequent use of the reducing equivalents to synthesize energy-rich compounds, such as hydrogen or fuels based on reduced carbon is the main objective of our research. In order to establish the design principles for a tandem, photochemical cell, we are assembling photoelectrodes that model photosystems I and II (PSI and PSII) of plants. The photoanode model of PSII contains colloidal IrO2 as water oxidizing catalyst, and a bioinspired system (BiP-PF10) consisting of a high oxidation potential porphyrin covalently attached to a benzimidazole-phenol pair (BiP) that mimics the TyrZ-His190 pair in PSII. When the BiP-PF10 construct is attached to TiO2 nanoparticles and excited with visible light, it undergoes photoinduced electron transfer. Electrons are injected into the semiconductor and the corresponding holes are localized in the phenol but due to the strong electronic coupling between BiP and PF10 some spin delocalization into the porphyrin macrocycle occurs. Electrochemical studies show that the phenoxyl radical/phenol couple of BiP-PF10 is chemically reversible with a midpoint potential of 1.24 V vs. NHE and is therefore thermodynamically capable of water oxidation. High field EPR provides a clear spectroscopic picture of these processes and the results are in agreement with transient absorption measurements.The photoelectrode model of PSI is part of a Gratzel type cell, designed to boost the potential provided by the photoanode. It is sensitized by low potential phthalocyanines, which absorb light in the near IR region of the spectrum. Upon photoexcitation, these dyes are designed to inject electrons into semiconductors having sufficiently negative conduction bands to effectively drive the reduction of protons to hydrogen at a cathode. The semiconductor is electrically wired to a cathode suitable for hydrogen production: either a metal electrode or a hydrogenase-mimic modified electrode.

Bio-inspired photoelectrochemical cells for water splitting

247th National Meeting (2014)

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