MinimizingProduct Back Reactions with Inert CarrierGas Bubbling for Photoelectrochemical Water Splitting with H₂/O₂ Coevolution

Single-particle-type suspension reactors and other membrane-free reactor designs for photoelectrochemical (PEC) water splitting, although promising for low-cost H₂ production, suffer the drawback of H₂ and O₂ coevolution leading to product losses through back reactions. These back reactions need to be minimized to increase the solar-to-hydrogen (STH) efficiency. H₂/O₂ coevolution also raises safety considerations, and a large-scale facility should maintain the H₂ concentration below the lower flammability limit with O₂ to eliminate explosion hazards. Herein, inert gas bubbling through the electrolyte was investigated with a tandem semiconductor for light-driven PEC water splitting without applied electrical bias as a technique to mitigate back reactions and maintain the output H₂ safely below the lower flammability limit during coevolution. A planar structure of nanoparticulate-Pt/Ni/np⁺-Si/FTO/TiO₂ was fabricated into both wired-electrode and fully integrated configurations capable of unassisted solar water splitting, though additional UV bias increased the current density and gas production rates. The device was characterized in aqueous electrolyte from alkaline to neutral conditions to balance water-splitting activity and durability against corrosion, displaying strong stability in a pH 11 buffer solution. Moreover, the H₂ faradaic efficiency was increased from 59% without bubbling to >98% using inert carrier gas to increase the mass transfer of products to the gas phase. Integrated tandem photoelectrode performance indicated a trade-off in bubbling flow rate between decreased back reactions and decreased light absorption due to bubble reflectance.