Solar Driven CO₂ Reduction to CO Catalyzed by Mn-Complex supported on Carbon Nanohorn in an All Earth Abundant System

CO₂ conversion technology have been developed for carbon neutral society. A lot of catalysts to convert CO₂ to chemicals have also been developed. Our group also reported CO₂ reduction catalysts. In particular, Mn-complex catalyst supported on multi-walled carbon nanotubes (MWCNTs) can catalyze CO₂ reduction reaction to CO at low overpotential (ca. 100 mV) [1]. In addition to the electrocatalytic activity, solar-driven CO₂ reduction to CO using Ni-doped β-FeOOH as a anode and Si solar cell was also reported [2]. Here, we report a carbon support effect to enhance the electrocatalytic activity [3]. We used MWCNTs, carbon nanohorns (CNHs), graphene platelets (GNPs), carbon black (CB) and carbon nanofibers (CNFs). Electrocatalytic activities Mn-complex supported on various carbon supports were measured in CO₂ saturated aqueous 0.1M K₂B₄O₇ + 0.2M K₂SO₄ solution at an applied potential of -1.0 V vs Ag/AgCl. Among the various carbon materials, CNHs were the best supports to enhance the catalytic activity toward CO₂ reduction to CO. Faraday efficiency of CO was 87%. The superior activity was speculated to the high specific surface area and hydrophobic nature. The water-drop contact angle revealed hydrophobicity of CNHs and MWCNTs. It is expected that a superior local environment for highly selective CO₂ reduction can be provided by hydrophobic supports by keeping water away from the electrode surface. In contrast, hydrophilic carbon supports resulted in the increase of Faraday efficiency of H₂. Additionally, specific surface area of CNHs (410 m²/g) was larger than other carbon supports. It is concluded that these properties resulted in the enhancement of catalytic activity of Mn-complex.
Because CNHs was revealed to enhance the catalytic activity of Mn-complex for CO₂ reduction to CO, we demonstrated solar-driven CO₂ reduction to CO using Mn-complex supported on CNH as a cathode electrode, Ni doped β-FeOOH supported on single walled carbon nanotube (SWCNT) as an anode electrode and polycrystalline Si solar cell. A solar-to-CO conversion efficiency was 3.3 %.

[1] S. Sato et al., ACS Catal., 2018, 8, 4452-4458.
[2] T. Arai, et al., Chem. Commun., 2019, 55, 237-240.
[3] T. Nishi et al., ChemNanoMat 2021, 7, 596.