An Efficient Catalytic Interface Design Enabling General CO₂ Reduction in Strong Acids

Abstract

In the past two decades, carbon dioxide (CO₂) emissions have soared by ~2.6% per year. Electrochemical CO₂ reduction reaction (CO₂RR) to produce value-added products has received tremendous research attention in recent years. With research efforts across the globe, remarkable advancement has been achieved, including the improvement of selectivity for the reduction products, the realization of efficient reduction beyond two electrons, and the delivery of industrially relevant current densities. Compared with homogenous molecular catalysts, heterogeneous molecular catalysts are one of the most promising electrocatalysis models for CO₂ conversion. In this thesis, we focus on the rational design of catalytic interfaces for improved electrocatalysis, and the research context is divided into three parts:

(1) Molecular complexes are an important class of catalysts for the CO₂RR. However, selective CO₂RR in strong acids remains challenging due to competition with the hydrogen evolution reaction (HER). Peripheral functionalization is effective for tailoring the intrinsic activity of molecular catalysts, mostly attributed to the inductive effect or to stabilization of reaction intermediates. Here we report that peripheral functionalization of immobilized molecular complexes with quaternary ammonium groups can regulate the catalytic activity by repulsing hydronium and repelling isolated water and thus suppressing HER tuning the kinetics and mass transport surrounding active sites, enabling high-performance CO₂RR in strong acids. The positively charged and hydrophobic alkylammonium groups affect the migration of water and hydronium in the double layer while the covalent cationic configuration stabilizes catalyst-electrolyte interfaces, inhibiting HER over extended potential windows. The effectiveness of our strategy is evidenced by testing of metallophthalocyanines, showing significantly improved acidic CO₂RR compared to the pristine molecules. Dodecyl ammonium-functionalized cobalt phthalocyanine suppress the hydrogen Faradaic efficiency (FE) to <10% in pH≈0.5 media, while providing a single-pass conversion efficiency up to ~85%. The selectivity can be maintained at 90% even in Li⁺ solutions, which often exhibit poor proton shielding. Our study underscores the role of second-sphere structure for selective molecular electrochemistry.

(2) The (bi)carbonate formation in neutral or alkaline media severely limits the carbon and energy efficiency of CO₂RR. Concentrated alkali metal cations and polymer coatings are commonly employed to inhibit HER during acidic CO₂RR. However, these approaches often exacerbate salt precipitation or increase cell resistivity. Here we report that cationic carbon nanotubes (CCNTs) serve as universal additives to metal catalysts, enabling efficient CO₂RR in strongly acidic media. Using a representative catalyst, bismuth, we attain FE of up to 95% for HCOOH products in pH≤1 electrolyte. Notably, while organic cations typically suffer from poor proton shielding and inferior CO₂RR product selectivity in aqueous solutions, the CCNT additives permit its use as electrolytes for selective acidic CO₂RR. The higher solubility of organic cations further allows for their use at elevated concentrations, thereby improving long-term stability and energy efficiency. Our Quantum Mechanics based mechanistic analysis reveals that CCNTs physically mixed with metal catalysts alter the distribution of electric field at the electrolyte/electrode interfaces, which impedes HER by suppressing hydronium ions migration while promoting CO₂RR in strong acids. These findings highlight the potential of CCNTs as a versatile and effective additive for advancing acidic CO₂RR with improved selectivity, carbon efficiency, energy efficiency, and stability.

(3) Acidic CO₂RR is known to be a promising approach to overcoming CO₂ reactant loss in alkaline and neutral electrolytes. However, the proton-rich environment near the catalyst surface favors the hydrogen evolution reaction, leading to a low Faradaic efficiency and energy efficiency for products. Currently, surface modification/functionalization strategy can efficiently manipulate local microenvironments of the catalyst/electrolyte interface to significantly enhance CO₂RR performance. In light of this, we generalize the application of our developed modulation strategy and demonstrate its strong compatibility with various other molecular or metal catalysts. Dodecyl ammonium-functionalized metalloporphyrins, such as tin porphyrin enable a H₂ FE of <10% in strongly acidic media (pH≤1). While for metal catalysts, we utilize dodecyl ammonium-functionalized CCNTs as the universal additives, achieving efficient CO₂RR in strong acid. We attain FE of nearly 100% and 67% for CO and C₂H₄ products for the silver and copper oxide nanosheets. Furthermore, by utilizing the homemade CCNTs with benzimidazolium cation, we further realize efficient C₂H₄ production at 752 mA cm^-2 with an excellent FE of 77%. This study provides a new avenue to boost CO₂RR through the structural design of CCNTs.

Date of Award	11 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruquan YE (Supervisor)