Spike Current Induction by Photogenerated Charge Accumulation at the Surface Sites of Porous Porphyrinic Zirconium Metal-Organic Framework Electrodes in Photoelectrochemical Cells

Photoelectrochemical (PEC) capacitors have recently gar-nered increasing interest based on their charge accumulation and dissipation mechanisms, particularly with respect to spike and overshoot currents, and have therefore been investigated for biomedical applications, including nerve photostimulation and biomolecular sensing. Porous metal-organic frameworks (MOFs) are capable of accumulating large amounts of photo -generated charge at their surface sites, owing to their large surface areas, and therefore may have potential as a new mate-rial for use in PEC capacitors. To explore the PEC capacitor properties of MOFs, we performed transient photocurrent measurements using PEC cells comprising porphyrinic zirco-nium MOF (PZ-MOF) electrodes in a phosphate-buffered saline solution. We observed a clear growth and decay of the cathodic current during light irradiation and the generation of an anodic reverse current when the light was turned off, thus inducing spike and overshoot currents. However, no spike or overshoot currents were observed when excess oxygen was introduced into the electrolyte. These results indicate that PZ-MOFs have the ability for photogenerated charge accumulation at the surface pores near the interface between the PZ-MOF electrode and the electrolyte. Thus, we have confirmed that PZ-MOFs are a promising PEC capacitor material that may be used in future biomedical applications.