Designing Electrospun Composite Carbon Nanofiber (CNF) Photoelectrodes for Photoelectrochemical Transformation of Emerging Drinking Water Pollutants

Abstract

Novel nanomaterials, such as carbon nanofibers (CNFs), present a unique opportunity to advance photoelectrochemical drinking water treatment by integrating a photocatalyst to improve material properties and performance. To this end, we have fabricated electrospun CNF-TiO2 composite photoelectrodes with high surface area, conductivity, chemical stability, and mechanical strength for use in photoelectrochemical water treatment applications. The CNF-TiO2 physical, chemical, and electrical properties can be tailored to influence drinking water pollutant transformation pathways by selectively varying fabrication parameters (e.g. TiO2 content, carbonization temperature). Integrating the photocatalyst into the nonwoven CNF framework provides a material that transforms emerging organic pollutants with diverse chemical properties via photochemical (UV/Vis radiation), electrochemical (applied potential) and photoelectrochemical (UV/Vis radiation + applied potential) processes at circumneutral pH. The favored transformation pathway is primarily influenced by carbonization temperature, which controls the CNF-TiO2 electrical conductivity and crystal structures. Electrical resistance decreases almost logarithmically with increasing carbonization temperatures (450 to 1000 °C) to produce composites with more orderly carbon structure (graphitic) and more conductive photocatalyst (rutile TiO2). At higher carbonization temperatures (≥ 1000 °C), pollutants with diverse chemical properties (e.g. log Kow) rapidly sorb to the composite electrode surface. The effect of CNF-TiO2 composite structure on pollutant sorption dramatically overwhelms sorption effects due to pollutant chemical properties. At lower carbonization temperatures (< 1000 °C), composite properties provide minimal pollutant sorption. While composites processed at higher carbonization temperatures encourage direct electrochemical pollutant transformation at photoelectrode surface, lower carbonization temperatures suggest composites are well-suited for indirect photochemical pollutant transformation. Herein we present photoelectrodes carbonized at different temperatures and their performance in transforming model organic pollutants. Outcomes of this work will help identify the types and properties of next-generation photoelectrode materials that are most promising for improving photoelectrochemical cells purposed for drinking water treatment.