Enhancing Electrochemical Carbon Dioxide Reduction by Polymer-Encapsulated Cobalt Phthalocyanine through Incorporation of Graphite Powder

Abstract

Cobalt phthalocyanine (CoPc) has been extensively studied as a catalyst for the electrochemical reduction of CO2 to value-added products. Previous studies have shown that CoPc is a competent and efficient catalyst when immobilized onto carbon-based electrodes using a polymer binder, especially when immobilized with a graphitic carbon powder support to increase charge transport. In this study, we systematically explore the influence of incorporating graphite powder (GP) into a polymer-encapsulated CoPc on the system’s activity for the electrochemical reduction of CO2. We report a protocol for incorporating GP into CoPc/polymer/GP catalyst films that facilitates physisorption of CoPc to GP, leading to increased activity for CO2 reduction. We show that the activity for CO2 reduction increases with GP loading at low GP loadings, but at sufficiently high GP loadings the activity plateaus as charge transfer is sufficiently fast to no longer be rate limiting. We also demonstrate that axial coordination is still important even in the presence of GP, suggesting that CoPc does not fully coordinate to heteroatoms on the GP surface. We develop a set of optimized conditions under which the CoPc/polymer/GP catalyst systems reduce CO2 with higher activity and similar selectivity to previously reported CoPc/polymer films on edge-plane graphite electrodes. The procedures outlined in this study will be used in future studies to optimize catalyst, polymer, and carbon support loadings for other polymer-catalyst composite systems for electrocatalytic transformations.

The Influence of pH and Electrolyte Concentration on Fractional Protonation and CO2 Reduction Activity in Polymer-Encapsulated Cobalt Phthalocyanine

“…Catalyst loading was determined as previously described . After centrifugation, the graphitic pellet was digested by the addition of 15 mL of TraceMetal Grade 1 M HNO 3 .…”

“…Porphyrins and phthalocyanines have been studied as molecular electrocatalysts for the CO 2 RR, with cobalt phthalocyanine (CoPc) being a particularly well-studied catalyst due to its unique electron configuration and favorable binding energy to CO 2 . − CoPc easily adsorbs to carbon electrode surfaces and has been an important model system to study the mechanism of the CO 2 RR , as well as one of the rare molecular catalysts able to reduce CO 2 to methanol under specific reaction conditions. , Our particular interest in CoPc has focused on improving the selectivity of CoPc-catalyzed CO 2 RR over the competing HER by encapsulating the parent complex within coordinating polymers to control the local microenvironment. Our group and others , have shown that encapsulating CoPc within poly­(4-vinylpyridine) (P4VP) results in increased reaction activity and selectivity for the CO 2 RR over the HER, leading to an increase in Faradaic efficiency for CO production from 60% for CoPc to >90% for CoPc-P4VP and a four-fold increase in turnover frequency for CO production. ,, …”