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

Abstract

Polymer-encapsulated cobalt phthalocyanine (CoPc) is a model system for studying how polymer–catalyst interactions in electrocatalytic systems influence performance for the CO2 reduction reaction. In particular, understanding how bulk electrolyte and proton concentration influence polymer protonation and in turn how the extent of polymer protonation influences catalytic activity and selectivity is crucial to understanding polymer–catalyst composite materials. We report a study of the dependence of bulk pH and electrolyte concentration on the fractional protonation of poly­(4-vinylpyridine) and related polymers with both electrochemical and spectroscopic evidence. In addition, we show that the fractional protonation of the polymer is directly related to both the activity of the catalyst and the reaction selectivity for the CO2 reduction reaction over the competitive hydrogen evolution reaction. Of particular note is that the fractional protonation of the film is related to electrolyte concentration, which suggests that the transport of counterions plays an important role in regulating proton transport within the polymer film. These insights suggest that electrolyte concentration and pH play an important role in the electrocatalytic performance for polymer–catalyst composite systems, and these influences should be considered in both experimental preparation and analysis.

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

“…In this work, we report a Cr­(2,2′;6′,2″-terpyridine)­Cl 3 complex with an appended 2,2′:5′,2″-terthiophene (TPTCrCl 3 ) that, when electropolymerized onto a glassy carbon electrode (GCE), forms a conductive p-TPTCrCl 3 redox polymer film that is active and selective for the electrochemical reduction of NO 3 – to NH 4 + via a cascade catalysis mechanism. The p-TPTCrCl 3 catalyst system is inspired by the nitrate reductase molybdoenzymes that reduce NO 3 – to NO 2 – via an oxygen atom transfer from NO 3 – to the Mo-containing active site buried within the enzyme scaffold with controlled substrate and proton transfer. − Similar to nitrate reductase, our p-TPTCrCl 3 system incorporates Cr active sites, with similar oxophilicity as Mo, inside of a polymeric structure that tunes the local microenvironment and controls the transport of substrate and intermediates to enhance reaction selectivity. − We have previously demonstrated the benefits of catalyst microenvironments by encapsulating CO 2 reduction electrocatalysts in polymeric scaffolds. , Notably, this catalyst system operates in the presence of phosphate, which can cause deactivation of many other heterogeneous systems for NO x reduction due to the competitive occupation of available active sites and pore blocking by phosphate. − The p-TPTCrCl 3 system reduces NO 3 – to NH 4 + with fast rates of 0.36 ± 0.01 mmol NH 4 + mg cat –1 h –1 with >90% Faradaic efficiency (FE) for NH 4 + at only −0.75 V vs the reversible hydrogen electrode (RHE). The p-TPTCrCl 3 catalyst system represents one of the first examples of a molecular catalyst, homogeneous or heterogenized, that operates with comparable activity and selectivity to state-of-the-art solid-state catalysts for NO 3 – reduction to NH 4 + .…”