Poly‐4,7‐Di‐2‐Thienyl‐2,1,3‐Benzothiadiazole Photocathode for Efficient Photoelectrochemical Hydrogen Peroxide Synthesis

Abstract Photocatalytic/photoelectrochemcial oxygen reduction reaction (ORR) in an aqueous solution offers a promising way for green hydrogen peroxide (H₂O₂) synthesis. Lots of photocatalysts/photoelectrocatalysts with high activity have been demonstrated up to now. However, the resulting H₂O₂ concentrations remain low (typically below 10 mmol L⁻¹), posing a significant challenge for effective accumulation. Here, it is reported that poly‐4,7‐Di‐2‐thienyl‐2,1,3‐benzothiadiazole (denoted as pS‐DBT) photocathode, an organic donor‐acceptor‐donor (D‐A‐D) based polymeric semiconductor with wide visible light response (bandgap ≈1.7 eV), generates 2e⁻ selectivity beyond 90% with moderate PEC ORR activity in alkaline solution. Impressively, it enables sustained synthesis and accumulation of H₂O₂ up to 123 mmol L⁻¹ (≈0.4 wt. %) at 0.65 V versus RHE under simulated visible light (100 mW cm⁻², λ ≥ 420 nm) for 13 h, which is 20% higher than the previously state‐of‐the‐art polyterthiophene (pTTh) photocathode. This improvement for the pS‐DBT is ascribed to a 37% lower decomposition rate regardless of a 20% lower production rate in comparison with those for the pTTh. This work demonstrates a key avenue to enhance steady‐state H₂O₂ concentration by inhibiting parasitic loss of H₂O₂ due to further reduction reaction during the production process.