Tagliabue, GiuliaMa, Jiaming2024-10-022024-10-022024-10-02202410.5075/epfl-thesis-10571https://infoscience.epfl.ch/handle/20.500.14299/241451The intermittent nature of renewable solar energy poses a significant challenge to the power grid, requiring technologies for energy storage. In this regard, integrating a photoelectro-chemical (PEC) cell with a redox flow battery (RFB) presents an attractive prospect due to its ability to decouple power and energy storage, along with fast reaction kinetics. This device, known as a solar redox flow battery (SRFB), enables bias-free solar energy conversion and simultaneous electrochemical energy storage. Despite intensive research efforts in this field, which involve either semiconductor-based photoelectrode modes or integrated solar cell modes, significant questions remain regarding the scalability of these demonstrated devices for real-world deployment of this emerging technology. Indeed, realizing optimized SRFB systems is challenging, as it requires sufficient utilization of the solar spectrum, attainment of reasonable cell voltages, efficient mass transport, superior charge transfer, along with an economically feasible design. This thesis explores different aspects regarding SRFBs optimization: (i) synergistic morphology and photonic design of wide bandgap semiconductor-TiO2, towards optimized light absorption and enhanced charge separation; (ii) facile synthesis of high-performance hematite nanostructures for easy scalability and high state of charge (SOC), paving the way for large-scale deployment; (iii) heterojunction engineering compatible with upscaling nanofabrication techniques for further enhancing charge transfer; (iv) realizing enhanced mass transport and thus improving PEC performance via cell design. In the first part of this thesis (Chapter 2), we explore how the synergistic coupling of plasmonic and micro-/nano engineering can optimize the performance of photoanodes for solar powered redox cells (SPRCs). The plasmonic nanoparticles can simultaneously contribute to enhanced light absorption, improved interfacial charge carrier transfer as well as photogenerated charge carriers at sub-bandgap photon energies. In the second part of this thesis (Chapter 3), to circumvent the limited solar spectrum absorption of TiO2, we target the development of earth-abundant and scalable photoanodes. Specifically, we develop a high-performing hematite photoanode and demonstrate its potential using an integrated SRFB, with optimized mass transport. Thanks to an optimal band alignment with the redox couples and efficient mass transport, the demonstrated hematite photoanode can achieve unassisted photocharging to SOC higher than 50% and an overall solar-to-output energy efficiency (SOEE) at 0.11%. Importantly, our simple fabrication method makes it possible to easily upscale the photoanode as well as the cell size. To further improve charge separation both within the bulk and at the interface, in the third part of this thesis (Chapter 4), we present a scalable, nanostructured α-Fe2O3/CuxO p-n junction and demonstrate its largely improve unassisted photocharging of an integrated SRFB. In addition, in the Appendix to this thesis (Supporting Information D), we report the preliminary design of a microfluidic SRFB with improved mass transport. Overall, the results of this thesis provide an in-depth understanding of the interplay between light absorption, charge transfer and mass transport in SRFBs. More broadly, the results presented in this thesis contribute to the real-world deployment of this promising technology.ensolar redox flow batterysemiconductorlight absorptioncharge transfermass transportband alignmentplasmonicnano-engineeringheterojunctionscalabilitythesis::doctoral thesis