Improved stability and selectivity of CuFe2O4-based catalysts for photoelectrochemical CO2-to-acetate conversion

Abstract

It is well known that photocathodes using CuFeO2-based catalysts produce acetate via photoelectrochemical (PEC) CO2 reduction. However, these catalysts suffer from low stability; the PEC activities rapidly lower during the operation. To solve this problem, we developed a CuFe2O4-based catalyst. The photocathodes using the new catalyst achieved significantly higher stability for acetate production along with higher Faradaic efficiencies (FE) exceeding 90%, compared with the conventional CuFeO2-based photocathodes. Operando XAFS analyses and complementary techniques revealed that a considerable fraction of the Cu species were reduced to metal Cu in CuFeO2 while the Fe species were leached into the electrolyte during the PEC operation, which is the cause of the low stability. By contrast, CuFe2O4 was stable; no change in the oxidation state of Cu or no leaching of Fe was observed. A mixture of CuFe2O4 and CuO increased the current density compared with that of the single-phase CuFe2O4 calatyst. Spectroscopic analyses of electronic states determined the energy levels of CuFe2O4 and CuO, indicating that CuFe2O4 could possess higher FEs than CuFeO2, and that electrons photoexcited in CuO also could be consumed for acetate production after migrating to CuFe2O4. Thus, CuO functioned as a conductive additive, and increased the photocurrents while maintaining the high FEs. However, long-term stability of CuO was not sufficiently high. Hence, replacements of CuO with a more stable conductive additive is needed for fully exploiting the high stability and selectivity of CuFe2O4.