Product Distributions and Efficiencies for Ethanol Oxidation in a Proton Exchange Membrane Electrolysis Cell

Abstract

The stoichiometry, efficiency, and product distribution for ethanol oxidation in fuel cell hardware has been determined at 80 • C for commercial Pt/C, PtRu/C and PtSn/C anode catalysts. The amounts of ethanol consumed and acetic acid and acetaldehyde produced were determined by proton NMR spectroscopy while CO 2 was measured with a non-dispersive infrared CO 2 monitor. The Pt/C catalyst was most selective for the complete oxidation of ethanol to CO 2 at all potentials and therefore produced the highest number of electrons per ethanol molecule (stoichiometry). Consequently, it would provide the highest efficiency for a fuel cell, and for an electrolysis cell at high current densities. However, PtRu/C provided much higher currents at low overpotentials and therefore better electrolysis efficiency than Pt/C at low current densities. The main product at the PtRu/C catalyst was acetic acid, with ≥ 86% conversion at potentials ≥ 0.35 V vs. a dynamic hydrogen electrode. The PtSn/C catalyst also provided high yields of acetic acid (65-75%), with substantial production of CO 2 (26-27%) at high potentials. The electrochemical oxidation of ethanol in cells with proton exchange membrane (PEM) electrolytes is of central importance to the development of energy technologies based on bio-ethanol. PEM-based direct ethanol fuel cell (DEFC) 1,2 power systems are potentially one of the best alternatives to internal combustion engines, 3 although their efficiencies will need to be increased significantly. Alternatively, ethanol electrolysis cells (EEC) can be used to produce hydrogen. [4][5][6] In an EEC, ethanol is oxidized at the anode and protons are reduced to hydrogen at the cathode. In a DEFC, ethanol is oxidized at the anode and oxygen is reduced to water at the cathode. The attraction of these technologies arises from their high theoretical efficiencies and the perception that emissions will be low. However, the incomplete oxidation of ethanol at all known catalysts currently results in low efficiency cells and large amounts of byproducts.2 To achieve high energy efficiencies in EECs and DEFCs the faradaic efficiency (ε F ) for the electrochemical oxidation of ethanol to carbon dioxide (Eq. 1) must be high.2,7 However, in practice low yields of CO 2 have generally been reported, with the major products being acetaldehyde (Eq. 2) and acetic acid (Eq. 3).The faradaic efficiency is the ratio of the average number of electrons transferred per molecule of ethanol (n av ) to the maximum of 12 for the complete oxidation to CO 2 (ε F = n av /12). It is determined by the product distribution according to Eq. 4,where n i is the number of electrons transferred to form product i and f i is the fraction of ethanol converted to product i. The importance of n av in determining the efficiency of ethanol oxidation technologies makes product analysis of central importance in the development of better anode catalysts. 9Although there have been many advances in the electrochemical performances (voltage efficiency) of ethanol oxidation catalysts...

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.…”

“…4,7 n av = n i· f i [4] where n i is the number of electrons transferred to form product i and f i is the fraction of ethanol converted to product i (CO 2 , acetic acid and acetaldehyde account for >99% of the ethanol consumed 8 ). Various experimental methods are available for measuring n av , including analysis of the amount of ethanol consumed, 9,10 the product distribution, 7,10 and several electrochemical methods.…”