Pt and PtRu catalyst bilayers increase efficiencies for ethanol oxidation in proton exchange membrane electrolysis and fuel cells

Abstract

Polarization curves, product distributions, and reaction stoichiometries have been measured for the oxidation of ethanol at anodes consisting of Pt and PtRu bilayers and a homogeneous mixture of the two catalysts. These anode structures all show synergies between the two catalysts that can be attributed to the oxidation of acetaldehyde produced at the PtRu catalyst by the Pt catalyst. The use of a PtRu layer over a Pt layer produces the strongest effect, with higher currents than a Pt on PtRu bilayer, mixed layer, or either catalyst alone, except for Pt at high potentials. Reaction stoichiometries (average number of electrons transferred per ethanol molecule) were closer to the values for Pt alone for both of the bilayer configurations but much lower for PtRu and mixed anodes. Although Pt alone would provide the highest overall fuel cell efficiency at low power densities, the PtRu on Pt bilayer would provide higher power densities without a significant loss of efficiency. The origin of the synergy between the Pt and PtRu catalysts was elucidated by separation of the total current into the individual components for generation of carbon dioxide and the acetaldehyde and acetic acid byproducts.

Determination of the Stoichiometry of Ethanol Oxidation from the Flow Rate Dependence of the Current in a Proton Exchange Membrane Electrolysis Cell

“…This avoids inaccuracies that arise due to the chemical reaction of ethanol with oxygen when measurements are made in a DEFC. 15 Chemical analysis of the cell exhaust by nuclear magnetic resonance (NMR) and infrared (IR) spectrometry has been reported F480 Journal of The Electrochemical Society, 165 (7) F479-F483 (2018) previously for the same anodes, 6,8 with the accuracy of n av verified by the mass and charge balances, and agreement of values obtained from the amount of ethanol consumed and the product distribution.…”

“…6,8 Catalyst suspensions, prepared by dispersing the catalyst in a 1:1 mixture of 1-propanol and Nafion solution (Dupont; 5% Nafion) by sonication for 3 h, were spread onto Toray carbon fiber paper (CFP; TGP-H-090). Metal loadings of 3.2 mg cm −2 and Nafion loadings of ca.…”

“…6,8 This included flushing the cell with water, operating with ethanol at 0.7 V for at least 2 h and then recording polarization curves several times between 0.1 V and 0.7 V (0.05 V steps for 300 s). When applying the electrochemical method (Eq.…”

“…The procedures for determining n av by chemical analysis have been previously described. 6,8 The cell was operated at constant potential with an ethanol flow rate of 0.2 mL min −1 . CO 2 was measured in real time with a commercial non-dispersive infrared CO 2 monitor (Telaire 7001), while residual ethanol, acetic acid, and acetaldehyde in a sample of the liquid exiting the cell were measure by proton NMR spectrometry.…”

“…Since the overall efficiency of a DEFC is proportional to the number of electrons transferred, 3 measurement of the stoichiometry of ethanol oxidation (average number of electrons transferred per molecule, n av ) plays a crucial role in the development of more efficient anode catalysts for DEFCs. 3,6 Simple and fast methods are required for routine evaluation of the efficiency of catalysts over a wide range of conditions.The faradaic efficiency (ε F ) of a DEFC is determined by the ratio of n av to the maximum of n = 12 for the complete oxidation of ethanol to carbon dioxide (ε F = n av /12), while n av is determined by the product distribution according to Eq. 4,…”