The increasing demand for renewable energy integration necessitates efficient energy storage solutions to address the intermittency of sources like sunlight. Photoelectrochemical (PEC) processes, while attractive for solar energy storage, are hindered by slow kinetics and high costs. Redox flow batteries (RFBs), offering safety advantages and independent energy capacity and power rate, are a promising alternative. Integrating RFBs with PEC processes, creating solar rechargeable redox flow batteries (SRFBs), offers a compelling approach. Although theoretical studies suggest high solar-to-chemical conversion efficiency even with single-photon devices, practical implementations are limited by slow redox kinetics at the photoelectrode surface. Previous studies, often adapting architectures from solar water splitting research, have reported significant overpotentials. This study aims to understand the fundamental aspects of photoelectrode-electrolyte interfaces concerning conductivity and energy level matching to maximize photo-charging efficiency (solar-to-chemical efficiency, STC%). The researchers investigate the impact of different conducting layers on charge transfer and overpotential in a c-Si photoelectrode system, aiming to provide design principles for enhanced efficiency in SRFBs.

The literature review highlights existing challenges in solar energy storage, comparing PEC processes with other methods. The authors discuss the limitations of conventional PEC fuel conversion due to sluggish kinetics and the cost of energy storage, while acknowledging the safety and scalability advantages of redox flow batteries (RFBs) over Li-ion batteries. The review notes the recent interest in SRFBs, though existing SRFB demonstrations suffer from slow kinetics at the photoelectrode surface. Prior work using various semiconductor materials and architectures, often adapted from water-splitting research, is examined. The review underscores the need for a deeper understanding of the photoelectrode-electrolyte interface to improve charge transfer and reduce overpotentials, ultimately improving the solar-to-chemical conversion efficiency.

The study employed electrochemical linear sweep voltammetry (LSV) to investigate the electrochemical reduction reactivity of different conducting materials (Pt and carbon) sputtered onto highly doped silicon (c-Si) substrates, both with and without a TiO2 protective layer. Mott-Schottky analysis was used to investigate the impact of these conducting layers on the band structure. Quartz crystal microbalance (QCM) was used to monitor the thickness of the carbon layers during deposition. Atomic force microscopy (AFM) was employed to characterize the surface morphology and coverage of the deposited carbon layers. Photoelectrochemical charging tests were performed using both three-electrode and two-electrode configurations with various redox couples (ferri-/ferrocyanide, TEMPO-sulphate, Cu2+/+, NH4Br). The performance of the photoelectrodes (n-type and p-type c-Si with different conducting layers) was assessed using cyclic voltammetry (CV) and LSV under simulated AM1.5G illumination. The solar-to-chemical conversion efficiency (STC%) was calculated using a cell voltage at a given state of charge (SOC). Theoretical modeling based on the Shockley-Queisser limit was employed to compare experimental results and analyze efficiency losses.

The researchers observed a significant material dependency in the redox reaction kinetics, even with the simple single-electron transfer reaction of ferri-/ferrocyanide. The bare c-Si showed poor reactivity, attributed to surface deactivation. Pt and carbon layers improved reactivity, though the combination of carbon with TiO2 resulted in unexpectedly poor kinetics due to poor carbon coverage. LSV results showed that improved carbon coverage (achieved through controlled deposition parameters) led to significantly enhanced electrochemical activity, surpassing even optimized Pt films. This finding highlights the importance of surface coverage for minimizing substrate oxidation and facilitating charge transfer. AFM analysis confirmed the correlation between carbon coverage and deposition time, revealing the formation of carbon islands whose size increased with deposition time. The study also found that the ‘pinch-off’ effect (electrostatic screening of the substrate) plays a significant role, influencing charge transfer based on the size of the conducting layer islands relative to the silicon depletion width. Photoelectrochemical measurements using a back-illuminated pn+-Si photoelectrode demonstrated a high photocurrent of ~35 mA cm⁻² at 0% SOC, attributed to reduced optical losses from direct light absorption. A solar-to-chemical conversion efficiency of over 9.4% was achieved at 10% SOC for a Cu-sulphate||ferrocyanide cell, exceeding previously reported efficiencies for single-photon SRFBs. The comparison of experimental and modeled results indicated a considerable room for improvement by reducing the overpotential and series resistance. The study highlights the importance of band alignment design and optimized surface coverage to minimize the charge-transfer barrier. It also identified challenges relating to the precipitation of copper compounds in some configurations, suggesting further improvements in the RFB cell design might be necessary.

The findings directly address the research question of optimizing photoelectrode design for SRFBs by demonstrating the critical role of band alignment and surface coverage control in minimizing overpotentials and maximizing charge transfer. The high efficiency achieved (9.4% at 10% SOC) represents a significant advancement in SRFB technology. The discrepancy between experimental and modeled efficiencies points to areas for future improvements, such as reducing series resistance and optimizing the cell voltage to maximize average STC% over the entire charging cycle. The results provide crucial insights into the interface design principles needed for SRFB development, potentially applicable to other PEC systems. The challenges encountered with copper precipitation indicate the need for further optimization of the redox couples and cell design. The findings advance the understanding of SRFBs and guide further research towards more efficient and practical solar energy storage solutions.

This study reveals crucial design principles for efficient photoelectrodes in SRFBs. The achievement of over 9.4% solar-to-chemical conversion efficiency with a single-photon device highlights the significant potential of the proposed band alignment and surface coverage strategies. Future research should focus on reducing series resistance, optimizing cell voltage for maximized average STC%, and mitigating issues such as copper precipitation to enhance overall performance and stability. The findings offer significant insights into the development of highly efficient and practical SRFBs for solar energy storage.

The study primarily focuses on the photoelectrode design and optimization, with limited exploration of the overall SRFB cell optimization. The observed copper precipitation issue, although discussed, wasn’t fully addressed. Long-term stability and durability testing of the optimized photoelectrodes remain to be explored comprehensively. Further, the theoretical model used might not fully capture all aspects of the complex processes occurring at the photoelectrode-electrolyte interface.