The synthesis of carbon dots for possible use as a carbon monoxide sensor in polymer electrolyte membrane fuel cells

Abstract

Carbon monoxide (CO) poisoning is one of the main factors retarding the advancement of the commercialisation of polymer electrolyte membrane (PEM) fuel cells because it drastically decreases the efficiency of the fuel cell through the poisoning of Pt. To date, carbon dots (CDs) have not been explored for sensing CO for this application. CDs are carbon nanomaterials of less than 10 nm diameter and with unique photoluminescence properties. Herein, we investigate the synthesis of CDs for possible use as CO sensors in PEM fuel cells. To develop a synthesis method, three reagents were investigated as carbon precursors for the CDs synthesis namely, sucrose, chitosan and polyethylene glycol (PEG). Furthermore, use of three different heat sources was investigated for the hydrothermal synthesis of CDs namely, the autoclave, the microwave and the reflux methods. PEG was found to be the best precursor for CDs synthesis and the autoclave method was favoured because it is a more facile and reliable synthesis method. Hence, the synthesis of CDs was investigated further through the synthesis of CDs made from PEG using the autoclave method while varying other reaction parameters. The variation of the PEG (1) precursor concentration, (2) precursor molecular mass and (3) the reaction time were all found to have no significant effect on the chemical nature and size of CDs. It was found that long reaction times, beyond 4 hours, produced carbon nanospheres (CNSs) as by-products. The production of CNSs as by-products was found to prevail when water was used as a solvent. The variation of the solvent used was found to have a huge impact on the particle size and chemical nature of the resulting CDs, the size, quantum yield (%QY) and whether the CDs made were short- or long-wavelength CDs. Solvents with higher boiling points produced smaller CDs. Solvents with high relative polarity were found to yield CDs with a high %QY. Short-wavelength emitting CDs were favoured using solvents with non hydrogen bonding properties, low boiling point, low relative polarity and low molecular weight. In general, the %QY was found to correlate with the particle size of the synthesised CDs with exceptions where aggregation induced emission was at play. The most photoluminescent synthesised CDs had a %QY of 15% (2.04 nm sized CDs) and 18% (1.83 nm sized CDs) from reaction time studies (3 hours) and a molecular mass study (PEG6000), respectively. Preliminary tests on CDs were conducted as proof of concept, where CDs were successfully incorporated into a filter paper to produce a highly photoluminescent paper-based sensing device. Changes in photoluminescence (PL) were monitored on the paper using a UV-lamp. iv Exposure to Pt switched off the PL of the CDs/paper-based sensor and no PL change was observed upon CO exposure to pure CDs. In conclusion, the preliminary results indicate a proof of concept approach to produce a sensing device using CDs that can be tailored for CO detection, that requires further study.