Covalently Grafting Graphene onto Si Photocathode to Expedite Aqueous Photoelectrochemical CO₂ Reduction

Abstract

Silicon semiconductor functionalized with molecular catalysts emerges as a promising cathode for photoelectrochemical (PEC) CO 2 reduction reaction (CO 2 RR). However, the limited kinetics and stabilities remains a major hurdle for the development of such composites. We herein report an assembling strategy of silicon photocathodes via chemically grafting a conductive graphene layer onto the surface of n + -p Si followed by catalyst immobilization. The covalently-linked graphene layer effectively enhances the photogenerated carriers transfer between the cathode and the reduction catalyst, and improves the operating stability of the electrode. Strikingly, we demonstrate that altering the stacking configuration of the immobilized cobalt tetraphenylporphyrin (CoTPP) catalyst through calcination can further enhance the electron transfer rate and the PEC performance. At the end, the graphene-coated Si cathode immobilized with CoTPP catalyst managed to sustain a stable 1-Sun photocurrent of À 1.65 mA cm À 2 over 16 h for CO production in water at a near neutral potential of À 0.1 V vs. reversible hydrogen electrode. This represents a remarkable improvement of PEC CO 2 RR performance in contrast to the reported photocathodes functionalized with molecular catalysts.

Covalent Functionalization of Silicon with Plasma-Grown “Fuzzy” Graphene: Robust Aqueous Photoelectrodes for CO2 Reduction by Molecular Catalysts

“…22 In these studies, the aromatic structure of CoPc enabled noncovalent π−π stacking on carbon supports, providing strong electronic interaction and fast electron transfer kinetics from the electrode to the catalytically active site. 19,23 For solar-driven PEC systems, sp 2 carbon electrodes have been integrated with a semiconductor light absorber in limited instances, 24 likely due to the low optical transparency of carbon and difficulty of achieving robust semiconductor/ carbon interfaces. One approach involves the transfer of highquality carbon onto silicon electrodes and patterning to enable light penetration.…”

“…For solar-driven PEC systems, sp 2 carbon electrodes have been integrated with a semiconductor light absorber in limited instances, likely due to the low optical transparency of carbon and difficulty of achieving robust semiconductor/carbon interfaces. One approach involves the transfer of high-quality carbon onto silicon electrodes and patterning to enable light penetration. − For instance, in a photovoltaic application using p -type Si ( p -Si), a p -Si/graphene solar cell was fabricated by transferring polycrystalline graphene onto Si and thereafter creating microscale holes to facilitate light penetration, yielding a current density of ∼31.56 mA/cm 2 for devices with a 12.8 μm holes under 1.5 sun illumination .…”