Selective CO2 Reduction with Catalyst-Assembled Electrodes in an Aqueous Solution
Authors: Naruta Yoshinori 
Published: February 2020

Suppression of CO2 emission is an urgent and important issue to be solved global warming problem. When one recycles emitted CO2 from burned fossil fuels as useful carbon resources, such as CO, the overall amount of emitted CO2 in air will be reduced by keeping the usefulness of carbon-containing fuels and materials as key chemicals in industries without increase of CO2 emission. On the design of CO2 reduction catalysts, overpotential (η), turnover frequency (TOF), and robustness are major issues for highly efficient catalytic reaction. Our strategy is based on mimicking the active site structures of metalloenzyms, carbon monoxide dehydrogenases (CODHs), which contain Ni-Fe bimetallic center in a Fe-S cluster.1 These enzymes can convert CO2-to-CO at a quite low η, < 100 mV.2 However, the enzymes easily lose their activity due to the lack of their robustness. On the other hand, reported metal complexes applied for CO2 reduction, different from the enzymes, involve single metal ion in their active centers and require relatively higher η
than those of the enzymes. On this context, we have developed efficient electrocatalysts and their reaction systems, which fulfill all essential requisites for catalytic CO2 reduction. We applied the bimetallic motif in the enzymes for the structures of CO2 reduction catalysts and synthesized Fe porphyrin dimers by linking two Fe porphyrins through a phenylene group.3 As expected, the catalytic activity is very sensitive on the Fe-Fe separation. As the results, we demonstrated homogeneous catalytic CO2-to-CO reaction with Fe meso-tris(perfluorophenyl)porphyrin dimer bearing o-phnylene linker in very high TOF, 25,000 s-1, at η = 0.42 V and 93% CO selectivity in a wet DMF. Controlled potential electrolysis under the same conditions keeps current density, ≈1 mA cm-2 for 12 h without any loss of the activity.4 Since CO2-to-CO reaction couples with water splitting, heterogeneous electrocatalytic system is important. When one uses a coordination catalyst-assembled electrode in an aqueous solution, following issues should be solved:
1. When one modifies a flat electrode with a coordination catalyst, catalyst coverage on the electrode is limited. In order to increase current density or reaction velocity, the surface coverage should be several orders higher than the modified flat electrode.
2. The standard reduction potential of proton is less negative than those of many CO2 reduction reaction. Thus, an electrode protected by a suitable material is required in order to avoid preferential proton reduction over CO2.
To overcome these issues, we use (1) nano tin oxide-sintered film on an electrode in order to increase the electrode surface area and (2) co-modification of a SAM layer for the protection of the metal oxide surface from water. By application of these methods, we attained selective CO2 reduction over hydrogen formation at zero overpotential in an aqueous solution in ten times higher current density than one with the ordinary flat electrode.
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