(583eb) Highly Efficient Photoelectrochemical Reduction of CO2 to Formate By Co3O4 Microflower-Nanorod Electrode

The deterioration of greenhouse effect, reasoned by the ever-increasing emission of CO₂ is one of the most alarming issues globally. In addition, the problem of energy shortage has also becoming more and more harshing. Artificial photosynthesis of C1 fuel from CO₂ in a photoelectrochemical manner has been a promising method to solve these problems. Among C1 fuels, formic acid is a typical and important intermediate in chemical industry and it could be combusted as liquid fuel, which could further applied in fuel cell. Producing formic acid from CO₂ could be a possible way to diminish the excess CO₂ in the environment and alleviate the shortage of fuel. However, the reported conversion efficiencies for CO₂ to formic acid were still rather low and far from commercially available. Co₃O₄, a typical spinel structured p-type semiconductor with outstanding reduction activity and a conduction band at -1.5 V, showed much feasibility and thermodynamic possibility in the photoelectrochemical reduction of CO₂. In the present paper, photoelectrochemical reduction of CO₂ to formate was realized on novel Co₃O₄ electrode with microflower-nanorod hetero-structured morphology, which was synthesized through one-step hydrothermal method. The petals of the microflower and side faces of nanorod were enclosed by {12-1} facet. Compared with common Co₃O₄ nanoparticle electrode, Co₃O₄ microflower-nanorod electrode showed superior cathodic photocurrent under visible light irradiation at -0.6 V (vs SCE). According to the Mott-Schottky characterization, higher carrier concentration was obtained on Co₃O₄ microflower-nanorod electrode thanks to the microflower-nanorod structure. Furthermore, large amount and high conductive octahedral Co³⁺ cations on the {12-1} surface also contributed to its high photocurrent response. In CO₂ saturated electrolyte, the photocurrent response of Co₃O₄ microflower-nanorod electrode also displayed the highest photocurrent, indicating its superior photoelectrocatalytic reduction activity toward CO₂. This was because a large amount of Co³⁺ existed on {12-1} surface. The Co³⁺ on the surface showed high reduction activity under negative bias. Besides, Co³⁺ cations were highly under coordinated on {12-1} surface, which facilitated the adsorption of CO₂ molecule onto Co₃O₄ surface. The above two aspects have contributed to the high reduction activity of Co₃O₄ microflower-nanorod electrode. In the photoelectrochemical reduction of CO₂ in 0.2 M NaHCO₃ solution, only formate was detected to in the liquid phase, reaching 132.2 μmol after 8 h’s visible light irradiation. The conversion efficiency was 0.66%, which was the highest efficiency compared with previous reports. Since the successful photoelectrochemical conversion of CO₂ to formate, the present paper has proposed a novel and applicable way for the artificial photosynthesis of C1 fuel from CO₂, which displays paramount importance.