Stand-alone photoconversion of carbon dioxide on copper oxide wire arrays powered by tungsten trioxide/dye-sensitized solar cell dual absorbers

A photoelectrochemical (PEC) cell composed of a WO₃/dye-sensitized solar cell (WO₃/DSSC) and copper oxide (Cu_xO, where x=1 and 2) wire arrays as a dual-absorber photoanode and cathode, respectively, is demonstrated as a stand-alone, durable device for CO₂ photoconversion. The Cu_xO wire arrays, which have high surface-to-volume ratios, exhibit promising electrocatalytic activity for CO₂ conversion to CO at Faradaic efficiencies of ~80% and ~60% at E=-0.2 and -0.4 V vs. RHE, respectively, and H₂ production is minimized at a Faradaic efficiency <~20% in the potential range between -0.2 and -1.0 V vs. RHE. The single-absorber cell of a WO₃ photoanode and Cu_xO wire array cathode couple (WO₃-Cu_xO) requires a minimum overpotential of ~0.7 V to drive CO₂ conversion. For stand-alone CO₂ conversion, a DSSC is coupled to the WO₃-Cu_xO system. In the dual-absorber cell (WO₃/DSSC-Cu_xO), the long-wave band (λ >ca. 450 nm) passed through the semitransparent WO₃ film is absorbed by the dye-sensitized TiO₂ electrode of the DSSC. The WO₃/DSSC-Cu_xO shows a potential gain of ~0.7 V and is able to successfully drive CO₂ conversion on Cu_xO and simultaneously oxidize water on WO₃ without an external power supply. In this stand-alone system, the primary CO₂ conversion product is CO, with a solar-to-chemical energy efficiency of ~2.5%; H₂ and formate are obtained with energy efficiencies of 0.7% and 0.25%, respectively, in 5 h (overall efficiency ~3.45%). Neither CO₂ conversion product nor H₂ is found using the single-absorber system.