The development of sustainable CO2-to-fuel conversion systems is a central issue in closing the global carbon cycle and producing renewable chemicals and fuels from the greenhouse gas CO2. However, efforts to demonstrate fuel production from CO2 powered by sunlight are currently hampered by the requirements of sacrificial electron donors or external bias, and lack of efficiency, selectivity and scalability. Ultimately, the CO2 reduction reaction (CO2RR) must be coupled with sustainable oxidation chemistry with an abundant electron source such as water for such technologies to satisfy global energy demands.
This work provides a novel approach to overcome those barriers and reports the first example of scalable and selective solar-driven CO2RR conjugated with water oxidation. A selective molecular catalyst was integrated on semiconductor light absorbers to form a wireless and monolithic photocatalyst sheet. Taking a phosphonated cobalt(II) bis(terpyridine) catalyst (CotpyP) modified SrTiO3:La,Rh|Au|BiVO4:Mo sheet as an example, the device combines the high selectivity of molecular catalysts for CO2 reduction and the strong water oxidation power of semiconductors. Hence, it provides a solar-to-formate conversion efficiency (STF) of 0.08±0.01% with selectivity for formate of 97±3%, setting a new benchmark in the field of wireless CO2 conversion using water as an electron donor. A benefit of the photocatalyst sheet configuration is that CO2RR and water oxidation occur in close proximity and the device, therefore, overcomes scale-up limitations inherent to (photo)electrochemical cell designs, where pH gradients and IR drop generated between the (photo)anode and (photo)cathode limit performance. Consequently, a comparable STF was observed even if the active area was increased 20 times.
This study overcomes the hurdle of current molecularly engineered systems requiring sacrificial reagents in photocatalytic CO2RR constructs, providing the impetus to study other molecular catalysts for solar fuel production via artificial photosynthesis in such architectures. The simplicity of the assembly process for integrating molecular catalysts in the device also allows the exploration of a wide range of catalysts for various photosynthetic reactions beyond CO2 utilisation to produce diverse products in the future.
