Stabilizing *CO₂ Intermediates at the Acidic Interface using Molecularly Dispersed Cobalt Phthalocyanine as Catalysts for CO₂ Reduction

Abstract

CO₂ electroreduction (CO₂ R) operating in acidic media circumvents the problems of carbonate formation and CO₂ crossover in neutral/alkaline electrolyzers. Alkali cations have been universally recognized as indispensable components for acidic CO₂ R, while they cause the inevitable issue of salt precipitation. It is therefore desirable to realize alkali-cation-free CO₂ R in pure acid. However, without alkali cations, stabilizing *CO₂ intermediates by catalyst itself at the acidic interface poses as a challenge. Herein, we first demonstrate that a carbon nanotube-supported molecularly dispersed cobalt phthalocyanine (CoPc@CNT) catalyst provides the Co single-atom active site with energetically localized d states to strengthen the adsorbate-surface interactions, which stabilizes *CO₂ intermediates at the acidic interface (pH=1). As a result, we realize CO₂ conversion to CO in pure acid with a faradaic efficiency of 60 % at pH=2 in flow cell. Furthermore, CO₂ is successfully converted in cation exchanged membrane-based electrode assembly with a faradaic efficiency of 73 %. For CoPc@CNT, acidic conditions also promote the intrinsic activity of CO₂ R compared to alkaline conditions, since the potential-limiting step, *CO₂ to *COOH, is pH-dependent. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts and devices for acidic CO₂ R.