Stable Unbiased Photo‐Electrochemical Overall Water Splitting Exceeding 3% Efficiency via Covalent Triazine Framework/Metal Oxide Hybrid Photoelectrodes

Abstract Photo‐electrochemical (PEC) water splitting systems using oxide‐based photoelectrodes are highly attractive for solar‐to‐chemical energy conversion. However, despite decades‐long efforts, it is still challenging to develop efficient and stable photoelectrodes for practical applications. Here, thin layers of covalent triazine frameworks (CTF‐BTh) containing a bithiophene moiety are conformably deposited onto the surfaces of a Cu 2 O photocathode and a Mo‐doped BiVO 4 photoanode via electropolymerization to construct new hybrid photoelectrodes, successfully addressing the efficiency and stability issues. The CTF‐BTh possesses a suitable band structure to form favorable band edge alignment with each metal oxide, creating a p–n junction and a staggered type‐II heterojunction with Cu 2 O and Mo‐doped BiVO 4 , respectively. Thus, the as‐fabricated hybrid photoelectrodes exhibit substantially increased PEC performances. Meanwhile, the CTF‐BTh film also serves as an effective corrosion‐resistant overlayer for both photoelectrodes to inhibit photocorrosion and enable long‐term operation for 150 h with only ≈10% loss in photocurrent densities. Furthermore, a stand‐alone unbiased PEC tandem device comprising CTF‐BTh‐coated photoelectrodes exhibits 3.70% solar‐to‐hydrogen conversion efficiency. Even after continuous operation for 120 h, the efficiency can still retain at 3.24%.