New Insights and Strategies for the Efficient Use of Conjugated Porous Polymers in Photoelectrochemical Solar Fuel Production

Abstract

Solar energy conversion into fuels such as hydrogen through photoelectrochemical (PEC) cells is an attractive way to solve the problems present in the actual energetic system. (1) Despite the advances that have been made in this line, it is still necessary to develop new materials and cell configurations to take this technology to a higher scientific level. The application of organic polymers in PEC is a hot topic that continues to grow due to the promising optoelectronic properties of this class of semiconductors. Conjugated polymers exhibit advanced properties because of it particular conjugation that confers it a huge conductivity through the whole structure. (2) In particular, Conjugate Porous Polymers (CPP) offers a higher photostability and robustness that is fundamental for long term application, but their synthesis often leads to a large-particle powder, unsuitable for preparing thin films, key to preparing high-quality thin films, limiting its applicability in PEC.(3)In this work, we present two innovative strategies that allow the efficient use of these materials in photoelectrochemical cells. The first consists of a synthetic route through mini or ultra-mini emulsion synthesis that provides particles from 500 nm to 20 nm, adequate for thin film preparation. The second consists in the design of suitable monomers to be electropolymerised, obtaining high quality thin films and highly controlled thickness. These strategies allow the complete characterization of the optoelectronic properties of the CPPs, since the thin films obtained are of high quality and stable over time. In this way, several polymers have been prepared and the electronic structure was determined by a combination different techniques such as XPS, electrochemistry and UV-VIS spectroscopy, revealing the suitability to be used as photoelectrodes.The photoelectrochemical properties were analyses by means of linear sweep voltammetry, chronoamperometry, and Photopotential measurements suggesting its potentiality for solar energy conversion. In addition, attending o the conductivity type and electronic properties, both photoelectrodes and tandem cells have been designed by combining them with different materials such as TiO2. The formed hybrid photoelectrodes have been characterized by X-ray diffraction, SEM, EDX and AFM. A series of photoelectrochemical measurements have been performed in a three electrode cell configuration, using the hybrid materials as working electrodes. (4)Hybrid photoelectrodes present improved photovoltages and photocurrents compared to TiO2 and CPP alone, confirming adequate light absorption and charge transfer between them. Besides, Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid photoelectrodes. Furthermore, the designed tandem cells were connected to a gas chromatograph and the hydrogen evolution was confirmed and quantified. Keywords: Photoelectrochemistry, Solar fuels, Organic materials, Conjugate Porous Polymers, References M. Grätzel, Photoelectrochemical cells. Nature. 414, 338–344 (2001).M. Liras, M. Barawi, V. A. de la P. O’Shea. Chem. Soc. Rev. 48, 5454–5487 (2019).M. Barawi, L. Collado, M. Gomez-Mendoza, F. E. Oropeza, M. Liras, V. A. de la P. O’Shea, C. Adv. Energy Mater. 18, 2101530 (2021).M. Barawi, E. Alfonso-González, C.G. López-Calixto, A. García, A. García-Sánchez, I. J. Villar-García, M. Liras, V.A. de la Peña O'Shea. Small, 18, 37. 2201351 (2022). Acknowledgements This work was supported by the Spanish national project NovaCO2 (PID2020-118593RB-C22) and ERC POC (NanoCPP). M.B thanks the Juan de la Cierva Incorporación grant (IJC2019 – 042430 –I).