Factors affecting selective electrocatalytic co 2 reduction with cobalt phthalocyanine incorporated in a polyvinylpyridine membrane coated on a graphite electrode

Abstract

Electrocatalytic CO 2 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 NaH 2PO 4 aqueous phase (pH 4.4), the catalyst membrane achieved a much more selective CO 2 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 .20V 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 CO 2 reduction were investigated, such as applied potential, pH and CoPc concentration in the membrane. An optimum applied potential of −1.20V vs. Ag¦AgCl was found for selective CO production. As for the pH, an optimum condition was obtained at pH ≈ 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/CO 2 intermediate accepts a third electron to produce CO.