Rational Design of Molecule-Support Interfaces for Efficient Electrocatalysis

Abstract

In the past two decades, atmospheric carbon dioxide (CO₂) concentrations have soared greatly. The electrochemical CO₂ reduction reaction (CO₂RR) has emerged as a promising approach for sustainable carbon cycling and valuable chemical production. With the effort of global researchers, various methods and strategies have been explored to boost CO₂RR performance. One of the most promising strategies is rational design of molecule-support interface. The interface is instrumental in adjusting adsorption behavior, influencing reactive intermediates, facilitating mass transporting, and suppressing the hydrogen evolution reaction. In this thesis, we focus on the rational design of molecular-support interfaces for improved electrocatalysis, and the research context is divided into 3 parts:

(1) Electrochemical CO₂ reduction reaction (CO₂RR) in acid improves carbon efficiency for advancing sustainable carbon cycling. While nickel single-atom catalyst (Ni-SAC) can efficiently convert CO₂ into CO in neutral or alkaline conditions, acidic CO₂RR is challenged by the competing hydrogen evolution reaction (HER). Here we demonstrate that modulating the local coordination structure of Ni-SACs can stall intrinsic HER activity across a wide potential, enabling efficient acidic CO₂RR. Our density functional theory calculations predict that Ni-SACs experiencing higher local curvature strengthen *COOH adsorption by 0.09 eV while increasing *H adsorption by 0.55 eV. To substantiate our simulations, we engineer Ni-SACs on carbon nanotubes (CNTs) of varying diameters, leveraging their intrinsic curvature to impose controlled local strain. Compared to Ni-SAC on 50-nm CNTs (Ni-CNT50), the 5-nm CNT one (Ni-CNT5) decreases hydronium and water reduction current density by 90 and 234 mA/cm² at -1.0 V and -1.8 V, respectively, in Ar-saturated 0.05 M H₂SO₄. In a flow cell with pH 1 catholyte, Ni-CNT5 maintains >95 % CO Faradaic efficiency (FE) from -1.0 V to -2.4 V, whereas Ni-CNT50 demonstrates ~70 % H₂ FE at -2.4 V. Thanks to the excellent HER suppression, Ni-CNT5 achieves 80 % single-pass CO₂ conversion efficiency and operates stably in acidic electrolyte with negligible activity or selectivity loss. Our work underscores how precise local coordination engineering in SACs facilitates efficient CO₂RR in acidic environments.

(2) One-dimensional cobalt-tetra-amino-phthalocyanine-based covalent organic polymers (1D-COP) with more distortions were developed by using single-walled CNTs (SWCNTs) as support to electrochemical CO reduction reaction (CORR). The COP on single-walled CNT (1D-COP/SWCNT) catalyst exhibits a maximum methanol Faradaic efficiency of 70 % in an H-cell, which exceeds those on wider-diameter multi-walled carbon nanotubes (22 % for 4-6 nm and 14 % for 10-20 nm). Using X-ray and vibronic spectroscopies, we have observed distinct local geometries and electronic structures induced by the strong interactions between the COP layer and the CNT substrates. Density functional theory calculations further support that increased curvature of the COP-SWCNT catalyst enhances the *CO binding species, leading to improved subsequent reduction reactions. Our results highlight the critical role of local structure in polymeric frameworks for improved electrocatalytic performance.

(3) Designing catalysts with controllable composition and morphology for the direct conversion of CO₂ into valuable products represents a sustainable approach to CO₂ recycling. In this work, two bimetallic organic covalent polymers (CuPc-AgBpy-COP/CNT and CuPc-AgBen-COP/CNT) based on (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato) copper (CuPc-8OH) were in-situ synthesized on CNTs support for electrocatalytic CO₂ reduction. Our experiments demonstrated that the bimetallic CuAg catalysts outperformed single-metal COPs. Notably, CuPc-AgBpy-COP/CNT achieved a CO₂-to-CH₄ conversion efficiency of 54.6 % at -1.6 V vs. RHE in H-cell, which was 3.7 times higher than CuPc-Bpy-COP/CNT and superior to the 42 % efficiency of CuPc-AgBen-COP/CNT. Additionally, CuPc-Bpy-COP/CNT exhibited the highest CH₄ partial current density at 7.2 mA/cm² at -1.6 V, indicating a synergistic effect of bimetals in the catalytic process. The presence of pyridinic-N facilitated the formation of CuAg nanocrystals while maintaining their size and distance. This work highlights that the microenvironment construction effectively influences the size and reconstruction of catalytic metals over the CNT, thereby enhancing CO₂RR toward desired products.

Date of Award	29 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruquan YE (Supervisor)