Factors affecting selective electrocatalytic CO2 reduction with cobalt phthalocyanine incorporated in a polyvinylpyridine membrane coated on a graphite electrode

Electrocatalytic CO2 reduction was carried out using a modified graphite electrode coated with a poly(4-vinylpyridine) (PVP) membrane containing cobalt phthalocyanine (CoPc). In a typical electrolysis in a 0.1 M NaH2PO4 aqueous phase (pH 4.4), the catalyst membrane achieved a much more selective CO2 reduction than that by a neat CoPc coating. The degree of the selectivity, defined as the ratio of CO to H-2 produced, was about 6 at -1.20 V vs. Ag\AgCl. The catalytic activity of CoPc was ascribed to the properties of PVP which is co-ordinative and weakly basic. Important factors affecting the CO2 reduction were investigated, such as applied potential, pH and CoPc concentration in the membrane. An optimum applied potential of -1.20 V vs. Ag\AgCl was found for selective CO production. As for the pH, an optimum condition was obtained at pH approximate to 5 where the PVP provides a partially protonated environment around the CoPc. The CO/H-2 selectivity was strongly dependent on the CoPc concentration in the membrane ([CoPc](PVP)) especially at lower applied potentials; higher [CoPc](PVP) brings about higher CO/H-2 selectivity. As for the effects of PVP, coordination of the pyridine group to CoPc as well as the microenvironment around CoPc provided by the polymer were concluded as dominant factors. A concerted protonation-deprotonation mechanism for the CO production process assisted by partially protonated PVP species was suggested. From the voltammetric and in situ potential-step chronoamperospectroscopy measurement, it was proposed that the CoPc/CO2 intermediate accepts a third electron to produce CO.