(Keynote) Solar CO2 Conversion into Liquid Fuels Via Thermodynamic and Kinetic Control
Authors: Kang Young Soo 
Published: December 2020

In this report, photoelectrochemical (PEC) system for the reduction of CO2 into liquid fuels of formaldehyde and acetaldehyde with minor hydrogen gas bubbling has been described in the aspect of thermodynamics and kinetics of the CO2 reduction reaction to have efficient method by lowering activation energy of CO2 reduction on the electron transfer reaction and to do reduction potential tuning of CO2 reduction reaction for the selective reduction products. Ca/Fe doped TiO2 photoanode oxidizes water and generates the large amount of O2, electrons and protons. On the other side, rGO(reduced graphene oxide)/PVP(poly(4-vinyl)pyridine)/Nafion multi-layers have been coated and fabricated on the surface of Cu foil cathode to reduce CO2 into acetaldehyde and formaldehyde compounds with different combined functions of each layer in the cathode. When solar light was irradiated on the surface of photoanode, electrons get excited to the conduction band of Ca/Fe doped TiO2 and transported to the cathode via external wire with low external bias potential. rGO on Cu foil has been used for the dual functions of reduction potential tuning and multi-electron shuttling process of CO2 reduction reaction. The multi-electron shuttling function was illuminated by larger number and longer life-time of excited electrons and generating electron cloud, which have been confirmed by time-resolved photoluminescence (TR-PL) decay curves and 2D time-resolved photoluminescence (TR-PL) mapping images of Cu/rGO electrode. This allows the sequential multi-electron transport process from Cu/rGOcathode to CO2, which was studied with time-resolved chronoamperometry and identification of the charge transfer complex formation with UV-vis spectrophotometer. The reduction potential tuning was controlled by the combined function of bias potential given by external wire and reduction degree of rGO to add proper amount of potential energy to transfer multi-electrons to CO2 in the thermodynamic aspect. Those combined functions are given to supply the tuned amount of energy into the open circuit potential of the system for photoanode of Ca/Fe doped TiO2 and Cu/rGO cathode to transport excited electrons to produce CO2 reduction product selectively. N-heterocyclic poly(4-vinyl)pyridine (PVP) helps to capture and do chemical activation of reactant CO2 molecule by complexation as [PVP-CO2*] complex via charge separation and lowering transition state energy level of the electron transfer for the formation of anion radical of carbon dioxide complex with PVP as [e- + PVP-CO2*]≠ complex via electron delocalization, which is basically possible due to high basicity of lone pair electrons of nitrogen atoms of N-heterocyclic PVP compounds. These functions result in the lowered activation energy for the CO2 reduction reaction to have faster kinetics. The decreased amount of activation energy was determined with Tafel plots by measuring the slopes of it and semicircles of the electrochemical impedance spectroscopy (EIS) Nyquist plots at different temperatures. Nafion film coated on the surface of cathode assists the faster transport of H+ ions to CO2 reduction reaction sites for the simultaneous one-pot reaction of proton coupled electron transfer reactions. It was checked with comparing the current density of the I-V curves of cathodes with and without Nafion layer coating of Cu foil cathode. Obtained results of CO2 reduction into liquid fuels via photoelectrochemical process show formaldehyde as major product with 25% current efficiency in Cu/rGO cathode and acetaldehyde as a major product with 35% current efficiency in Cu/rGO/PVP/Nafion as cathode during 4 h of photoelectrochemical reaction at 0.68 V vs RHE as an external bias potential.
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