Catalytic Electrodes for the Oxygen Reduction Reaction Based on Co-Doped Carbon Quantum Dots and Anion Exchange Ionomer
Authors: Vacandio Florence , Knauth Philippe , Syahputra Suanto 
Published: December 2024

The sluggish kinetics of the oxygen reduction reaction (ORR), more than six orders of magnitude slower than the hydrogen oxidation reaction in acidic conditions, is a major concern for various energy storage and conversion devices, including metal air batteries and fuel cells. The exceptionally high O––O bond energy (~500 kJ/mol) requires generally the use of noble-metal electrocatalysts, including platinum and palladium, for the four-electron reduction in acidic conditions.

The ORR is however faster in alkaline medium and non-noble electrocatalysts are applicable, including doped carbon materials. Nanostructured carbons have especially retained the attention due to their solubility in organic solvents that make the drop-casting procedure available. The electroactivity of carbon quantum dots (CQD) for the ORR was demonstrated in 2012.

We have recently compared the preparation of CQD by three different methods -pyrolysis, microwave irradiation and hydrothermal synthesis - and reported their respective properties. The hydrothermal method is most promising due to the easily controllable composition and structure via the optimization of precursors and can produce CQD with uniform particle size. We showed especially that the position of nitrogen atoms in the nanostructure, for example in graphitic, pyrrolic or pyridinic sites, changes significantly the properties of the CQD.

In this work, we therefore investigate a simple and easily up-scalable synthesis of heteroatom co-doped CQD, using inexpensive starting materials and a fast hydrothermal synthesis procedure, which provides uniform CQD size. The CQD are characterized by various techniques, including XPS, FTIR, and Raman spectroscopy.

The overpotential of the ORR contains the activation overpotential that can be reduced by the presence of a potent electrocatalyst improving the charge transfer kinetics, the Ohmic drop and the diffusion overpotential, related to the transport of oxygen gas and of hydroxide ions. Whereas the oxygen diffusion can be facilitated by the microstructure of the electrode, the hydroxide ion transport is enhanced by the presence of an anion exchange ionomer (AEI) in the electrode. The ORR occurs at the triple phase boundary between gas phase (pore), electron conducting catalyst (carbon paper/CQD) and hydroxide ion conductor (AEI), here poly(sulfone trimethylammonium hydroxide) (PSU-TMA).

The electrodes investigated in this work are therefore prepared by drop-casting on carbon paper of a slurry containing co-doped CQD and AEI. Impedance spectroscopy and d.c. capacitance measurements are used to determine the electrochemically active surface area of the electrodes. The electrocatalytic activity for the ORR is studied in alkaline conditions (1 M KOH) by cyclic (CV) and linear sweep voltammetry (LSV) at various speeds of a rotating disk electrode (RDE).

A.R. Nallayagari, E. Sgreccia, R. Pizzoferrato, M. Cabibbo, S. Kaciulis, E. Bolli, L. Pasquini, P. Knauth, M.L. Di Vona, Tuneable properties of carbon quantum dots by different synthetic methods, J. Nanostruct. Chem. 12 (2022) 565–580.

A.R. Nallayagari, E. Sgreccia, L. Pasquini, F. Vacandio, S. Kaciulis , M.L. Di Vona, P. Knauth , Catalytic electrodes for the oxygen reduction reaction based on co-doped (B-N, Si-N, S-N) carbon quantum dots and anion exchange ionomer, Electrochemica Acta 427 (2022) 140861.