Porous polyimides (pPIs) for metal-free electrocatalytic CO2 reduction

Abstract

Rising CO₂ levels due to human activities contribute significantly to global warming and
climate change, making the development of efficient CO₂ capture and conversion technologies
a critical global challenge. This project focused on the exploration of porous polyimides (pPIs)
for CO₂ capture and electrochemical conversion, with the aim of synthesizing and
characterizing three distinct pPI materials. Through a polycondensation reaction, three pPIs
(pPI-1, pPI-2, and pPI-3) were successfully synthesized, with pPI-3 being a novel material not
previously reported in the literature. The materials were characterized by FT-IR, UV-Vis-NIR
spectroscopy, PXRD, and BET surface area analysis, confirming their porous structure and
potential for CO₂ adsorption. Among these, pPI-2 demonstrated the highest surface area and
CO₂ uptake (97 m2
/g and 1.97 wt%), correlating with its superior electrocatalytic performance
for CO₂ reduction to formic acid (HCOOH). Electrochemical testing, including LSV, CV, and CA
studies, highlighted pPI-2’s consistent stability and high Faradaic efficiency (51.19%) in HCOOH
generation at more negative potentials, attributed to its larger pore size and optimized pore
volume, facilitating improved CO₂ diffusion and interaction with active sites.
In the co-solvent synthesis investigation, we examined the impact of different co-solvent
systems on the synthesis of pPI-1, with a focus on Hansen Solubility Parameters (HSPs), and
how the HSPs’ hydrogen-bonding component (δH) affect reaction yield and polymer quality.
The optimal hydrogen-bonding constant for pPI-1 synthesis was found to be δH = 7, where the
highest yield and best polymer quality were achieved, as confirmed by XRD data. The
introduction of zinc acetate as a catalyst proved crucial for achieving satisfactory yields, as
reactions without the catalyst showed negligible progress. Additionally, initial mechanistic
studies demonstrated the necessity of triethylamine (TEA) as a base to facilitate
polymerization, underlining its dual role in the reaction. XRD analysis further suggested that
the reduced π-stacking at δH = 7 could enhance the surface area of the polymer, offering
potential advantages for applications in catalysis.
In summary, this project successfully synthesized novel pPIs with promising characteristics for
CO₂ capture and electrocatalytic conversion, contributing valuable insights into the design and
synthesis of functional porous materials for environmental applications with potential
significant global impact.

Date of Award	13 May 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Charl F J Faul (Supervisor)