Development of phthalocyanine-based photoelectrodes for CO2 reduction and hydrogen production based on artificial photosynthesis

Increasing atmospheric carbon dioxide (CO₂) concentrations, the urgent need for sustainable energy sources, and global environmental and energy security challenges have raised significant concerns. CO₂ reduction via natural photosynthesis provides an important route for developing novel approaches. Among these, the photoelectrochemical (PEC) method is an ideal strategy for converting CO₂ into selective gaseous and liquid products (e.g., ethanol) under sunlight and ambient conditions. Many previous PEC CO₂ reduction studies have relied on expensive noble metals (Pt, Pd, etc.) as catalysts. To overcome these cost limitations, the development of affordable and abundant semiconductor catalysts is necessary. In this context, nickel oxide (NiO) catalysts possess unique properties that make them suitable for such applications; however, their wide bandgap remains a limiting factor in light-driven catalytic processes. To address this limitation, novel phthalocyanine derivatives, designed as biomimetic analogues of chlorophyll, were synthesized and characterized. Symmetric tetra-carboxylic acid substituted phthalocyanines containing iron or cobalt as the central metal atom were used to sensitize NiO electrodes. This approach enhanced light absorption, charge separation, and catalytic activity, thereby overcoming the inherent limitations of NiO for CO₂ reduction and water splitting. In this study, the photoelectrochemical CO₂ reduction performance and product analysis of nanoscale NiO nanofiber (NF) and nanocrystal electrodes were investigated. NiO-coated FTO electrodes were prepared with different coating layers, and CV, LSV, and EIS measurements were performed. The results indicated that three-layer NiO coatings provided optimal performance, and nanofiber electrodes exhibited higher current density, more positive reduction potentials, and lower charge transfer resistance (Rct) compared to nanocrystalline electrodes, demonstrating more efficient electron transport. Sensitization with phthalocyanine derivatives (C118 and C155) significantly enhanced catalytic activity under illumination. C118-modified electrodes exhibited the highest photoelectrochemical performance under both dark and light conditions. GC-MS analyses revealed that pristine NiO-NF electrodes showed CO and low-intensity ethanol peaks, while phthalocyanine-functionalized electrodes demonstrated significant increases in CO and ethanol signals over time. Notably, C118-sensitized electrodes displayed a rapid decrease in CO₂ signal alongside strong increases in CO and ethanol peaks, highlighting the effectiveness of phthalocyanine sensitization in promoting CO₂ reduction and selective product formation.