Engineering Interfacial Charge Transfer in TiO₂−g-C₃N₄ Heterojunction Photoanodes via Er Doping for Efficient Solar Redox Flow Batteries

Solar redox flow batteries (SRFBs) provide a promising platform for directly coupling solar energy conversion with electrochemical energy storage, but their performance is still limited by insufficient solar-spectrum utilization and sluggish interfacial charge transfer at photoelectrodes. Herein, an Er-modified TiO₂−g-C₃N₄ heterojunction photoanode was constructed by growing Er-doped TiO₂ nanorod arrays on fluorine-doped tin oxide followed by g-C₃N₄ deposition, and was evaluated in a TEMPO/VCl₃-based SRFB. Structural and spectroscopic characterizations show that Er modification preserves the main TiO₂ framework, promotes a nanorod-based heterostructure with intimate interfacial contact, and redistributes the optical response toward the visible−near-infrared region. Among the investigated electrodes, the 3 wt % Er:TiO₂−g-C₃N₄ photoanode exhibits the most favorable photoelectrochemical behavior, with the charge-transfer resistance decreasing from 28.55 to 23.82 Ω compared with the undoped TiO₂−g-C₃N₄ electrode. PL/TRPL and TPV measurements further indicate suppressed carrier recombination, with the TPV average lifetime increasing from 27.080 to 107.253 μs after Er modification. Wavelength-dependent IPCE and infrared-filtered photocurrent measurements reveal an enhanced but relatively weak long-wavelength photoresponse, suggesting that Er-related long-wavelength utilization acts as an auxiliary contribution rather than the sole origin of the improved performance. When assembled into a full SRFB, the optimized photoanode delivers stable unbiased photocharging currents of approximately 1.0−1.15 mA cm⁻² over repeated cycles, together with a solar-to-chemical conversion efficiency of about 1.15% and an estimated solar-to-output energy efficiency of 0.46−0.50%. Post-cycling SEM and XRD results further support the short-term structural stability of the photoanode. These results demonstrate that optimized Er modification primarily enhances interfacial charge separation and transfer in TiO₂−g-C₃N₄ photoanodes while providing an additional route for long-wavelength photon utilization in integrated solar conversion−storage systems.