Artificial Photosynthesis Using Iron Foam Photoanodes Coated with Catalytic Graphene-Manganese Porphyrin Self-Assembled Structures

Abstract

Photoelectrochemical (PEC) water splitting, also called artificial photosynthesis, is one way to produce clean hydrogen using solar energy to split water into oxygen and hydrogen gases. The main challenge in PEC field deals with low current densities of the photoanode. Today's conventional approaches to increase current densities are directed towards improving semiconductor (e.g. hematite) properties, increasing the active surface area of the electrode and adding known water splitting co-catalysts. The present work deals with the characterization of porous iron foams as photoanodes, instead of the mostly used planar transparent electrodes. The iron foams can be coated by a thin hematite layer by a simple heat treatment procedure. The performance of these electrodes is significantly increased by doping with silicon, Co(II) ions, and macrocyclic catalysts. The iron foams electrodes yield efficient photocurrents for water splitting in the range of ~ 0.2-0.4 mA/cm2 at 1.23 V vs. NHE. Anionic carboxylic-rich graphene oxide (CGO) and cationic Mn(III) tetrakis(N-Methyl-4-Pyridinium) porphyrin (MnTMPyP) form self-assembled leaf-like structures. These are obtained through electrostatic and π-π stacking interactions, as indicated by the red shift of the metalloporphyrin Soret band throughout the 6-12 pH range. When exploited for the photoelectrochemical oxidation of water, CGO-MnTMPyP films on iron/hematite foams exhibit substantial activity as evidenced by the current density (0.54 mA/cm2 at 1.23 V vs. NHE) and IPCE (9.0 % at 1.43 V vs. NHE) measured at pH 10.