Recent advances in photoelectrochemical redox flow cells, such as solar redox flow batteries, have received much attention as an alternative integrated technology for simultaneous conversion and storage of solar energy. Theoretically, it has been reported that even single- photon devices can demonstrate unbiased photo-charging with high solar-to-chemical con- version efficiency; however, the poor redox kinetics of photoelectrodes reported thus far severely limit the photo-charging performance. Here, we report a band alignment design and propose surface coverage control to reduce the charge extraction barrier and create a facile carrier pathway from both n- and p-type photoelectrodes to the electrolyte with the respective redox reaction. Based on these observations, we develop a single-photon photo- charging device with a solar-to-chemical conversion efficiency over 9.4% for a redox flow cell system. Along with these findings, we provide design principles for simultaneous optimisa- tion, which may lead to enhanced conversion efficiency in the further development of solar- rechargeable redox flow cells.@en

The transition from stable fossil fuel energy production to intermittent renewable sources poses a huge challenge to our generation. The mismatch between electricity supply and demand will become an increasingly
pressing problem, due to the growing share of renewable energy production and the electrification of the automotive industry. The upcoming field of Solar Redox Flow Batteries (SRFB) proposes a possible solution in the search for feasible energy storage. Integrating a photovoltaic module with a redox flow battery results in a highly reversible storage system, which has both the advantage of flexibility in capacity and charge/discharge power, as well as the direct and efficient coupling to solar energy generation. However, due to its early stage in development, the SRFB has several challenges to overcome, like low charging efficiencies and energy densities. To improve SRFB efficiency, this work provides a performance estimation model, to identify general sensitivities of SRFB efficiency with respect to the loss mechanisms associated with the device, such as (but not limited to) kinetic overpotentials and resistances, to establish device design principles. This model is also extended to simulate realistic environments, using real (daily/seasonally dependent) solar radiation and ambient temperature data. Moreover, a fit tool is realized to identify losses in a specific system, by varying parameters and mimicking experimental linear scan voltammetry data. Experimentally, several conducting layers are tested for their kinetics and conductivity and impact on device performance. A thin film of platinum is found to result in the best kinetics and lowest resistance. However, carbon-coated electrodes also show promising results and could be a feasible and cheaper alternative to platinum, when applying layer-optimization. Based on the design principles derived from the modeled and experimental results, a single-junction siliconbased SRFB is developed, based on a ferri-/ferrocyanide electrolyte, coupled with Cu2+=+, yielding high solar-to-chemical charging efficiency of 9.4%. This efficiency is close to the simulated efficiency, while multiple other SRFB photo-charging experiments follow modeled trends, validating the model.