Rational Design of Molecular Catalytic Interface for Improved Electrocatalysis

Abstract

In the past two decades, carbon dioxide (CO₂) emissions have soared by about 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. Among numerous CO₂RR catalysts, molecular complexes have attracted significant research attention because of their well-defined structure, which allows the establishment of a precise structural model for a better understanding of the CO₂RR mechanism. 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 molecular catalytic interfaces for improved electrocatalysis, and the research context is divided into 3 parts:

(1) A stable cationic molecular/carbon nanotube (CNT) interface was synthesized by diazo-reaction and methylation reaction. The methylation of cobalt (II) tetraamino phthalocyanine (CoTAPc) transforms the electron-donating amino groups into electron-withdrawing quaternary ammonium cations, which favor the formation of *COOH intermediate and the desorption of *CO, conducive to 130% increase of current density for CO₂RR. However, the catalysts leach severely, and consequently, the current density decays rapidly. To resolve the dilemma, we develop an in-situ functionalization strategy by first covalently grafting CoTAPc onto carbon nanotube (CNT) via a diazo-reaction, followed by a complete methylation reaction. This conduces to a 700% increase in carbon monoxide (CO) partial current density (j_CO) compared to that of the physically mixed sample at -0.72 V vs. reversible hydrogen electrode (RHE) with highly stable currents. In a flow cell, this covalently immobilized structure delivers an industrial-relevance current density of 239 mA cm^-2, CO selectivity of 95.6 % at 590 mV overpotential and very low molecular loading of 0.069 mg cm^-2. This work provides mechanistic insight and design strategy of charged molecular catalysts for high-performance and stable heterogeneous electrolysis.

(2) An axial covalently immobilized molecular/CNT surface was developed by a substitution reaction. The iron phthalocyanine (FePc) and iron tetra-nitro phthalocyanine (FeTNPc) molecular were covalently grafted onto carbon nanotube surface with O coordination by a substitution reaction at the metal center (FePc@CNT and FeTNPc@CNT). Compared to the FePc(II), which shows current decay due to strong CO adsorption and CO poisoning, the FePc(III)@CNT leads to significantly improved CO desorption and long-term stability toward CO2RR. The introduction of electro-withdrawing nitro substituents on FeTNPc(III)@CNT further improves the activity and selectivity of FePc for CO₂RR by tuning the electronic density at the Fe active site. In the flow cell, an outstanding CO₂RR activity with low onset potential (-0.25 V vs. RHE) and improved total current density (j_tot) and j_CO (-212.7 mA cm^-2 and -181.6 mA cm^-2) were achieved, together with good long-term stability for 10 h. This work provides a new strategy for the rational design of highly efficient CO₂RR electrocatalysts at the molecular level.

(3) A curved molecular/CNT with Å-scale molecular distortions interface was developed by using single-walled CNTs (SWCNTs) supports. The above studies show that molecular catalysts usually produce 2e^- transfer products (CO), and multi-electron products such as methanol (MeOH) have always been a research difficulty due to the involvement of multi-electron and proton transfer processes. Starting with CoPc, a model CO₂RR catalyst, we show that CNTs are ideal substrates for inducing optimum properties through molecular curvature. Using a tandem-flow electrolyzer with monodispersed CoPc on single-walled CNT (CoPc/SWCNT) as the catalyst, we achieve a methanol partial current density of >90 mA cm^-2 with a selectivity of >60%. CoPc on wide multi-walled CNTs (MWCNTs) leads to only 16.6% selectivity. We report X-ray spectroscopic characterizations to unravel the distinct local coordinations and electronic structures induced by the strong molecule-support interactions. These results agree with our Grand Canonical Density Functional Theory that calculates the energetics as a function of applied potential. We find that SWCNTs induce curvature in CoPc, which improves *CO binding to enable the subsequent formation of methanol, while wide MWCNTs favor CO desorption. Thus, we demonstrate that the SWCNT-induced molecular strain increases methanol formation. We also show that induced strain can accelerate the oxygen reduction reaction and CO₂RR for other catalysts. Our results show the important role of SWCNTs beyond catalyst dispersion and electron conduction.

Date of Award	7 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruquan YE (Supervisor)