Effect of Catalyst Layer Thickness and Porosity on Ethanol Oxidation
Authors: El Sawy Ehab N , Matchett Laura , Birss Viola Ingrid 
Published: February 2020

The ethanol oxidation reaction (EOR) occurs at the anode in both direct ethanol fuel cells (DEFCs) and in electrochemical alcohol breathalyzers. Acetaldehyde (AAL) and acetic acid (AA) are the main products of EOR at low temperatures, with the minor production of CO2. At low temperatures, both CO and AAL can cause catalyst surface poisoning and thus enhancing the oxidation or removal of AAL from within the catalyst layer should increase the catalyst efficiency and decrease its degree of poisoning. It is known that modifying the catalyst material (e.g., Pt) with a second metal or a metal oxide, can accelerate aldehyde oxidation. However, the investigation of the effect of the catalyst layer porosity and thickness on AAL removal and the EOR efficiency has not been studied in detail as yet.

In this study, surfactant-free Pt nanoparticles (NPs) were synthesized in-situ at Vulcan carbon (VC) and colloid imprinted carbon (CICs) supports in one step, aiming for 40% Pt loading, using ethanol as both the solvent and reducing agent. In order to study the effect of catalyst layer porosity, the CICs were synthesized in our lab using silica particles that were 12, 50, and 85 nm in diameter, to obtain carbon supports with an ordered porous structure and controlled pore size. Using the 40% Pt catalyst, a range of film thicknesses were studied, represented as 0.2 mgPt/cm2, 0.4 mgPt/cm2, and 0.6 mgPt/cm2, respectively. All of the electrochemical results were normalized to the real Pt surface area to enable the comparison of the porosity and film thickness with no interference that may arise from the differences in Pt NPs size or utilization.

According to the X-ray diffraction (XRD) analysis, the Pt NPs were of a similar size, namely 3.3-3.8 nm, indicating that the Pt NPs size is solely controlled by the synthesis method with almost no substrate effect. The transmission electron microscopy (TEM) images of Pt/VC, Pt/CIC12, Pt/CIC50, and Pt/CIC85 showed that the Pt NPs are homogeneously distributed at the surface of the VC and are located both within the pores of the CICs and on the outer surfaces of the CIC particles with a size that compares well with XRD analysis. Independent of the catalyst film thickness, a Pt electrochemical surface area of 40-50 m2/g and a Pt utilization of 45-60% were obtained, with a better Pt utilization as the specific area of the carbon substrate increase.

The oxidation of 1 M ethanol at Pt/VC catalyst showed a positive shift in the onset potential with increasing the catalyst film thickness, while no shift is observed in the case of Pt/CIC85. The superior performance of the CICs compared to VC is due to the better ethanol transport through and product transport out of the catalyst layer pores for the CICs, resulting in less Pt surface poisoning by EOR reaction products. In addition, when a rotating disc electrode was used, a negative shift in the onset potential was observed, also reflecting a less poisoned Pt surface due to the removal of the AAL product. More details about the behavior of ethanol oxidation, at both high and low concentration, and its dependence on Pt/C layer thickness and porosity will be discussed in the presentation.