Effects of crossover on product yields measured for direct ethanol fuel cells

James, David D.; Pickup, Peter G.

doi:10.1016/j.electacta.2010.02.007

Cited by 58 publications

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References 21 publications

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“…It is important to note that there is also the crossover of species from the cathode to the anode such as oxygen, ethanol and/or reaction products of ethanol oxidation that may accumulate at the cathode side of the membrane, which would increase by increasing the cathode backpressure. James and Pickup have discussed the issue of ethanol oxidation products (acetic acid and acetaldehyde) crossing to the anode side [23]. While Jablonski et al reported the effect of oxygen crossing from the cathode to the anode and reacting chemically [24] with ethanol to produce acetic acid and acetaldehyde.…”

Section: Effect Of Cathode Backpressure and Cell Temperaturementioning

confidence: 99%

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

Alzate

Fatih

Wang

2011

Journal of Power Sources

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Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance Alzate, V.; Fatih, K.; Wang, H. 196 (2011) a b s t r a c t A parametric study was conducted on the performance of direct ethanol fuel cells. The membrane electrode assemblies employed were composed of a Nafion ® 117 membrane, a Pt/C cathode and a PtRu/C anode. The effect of cathode backpressure, cell temperature, ethanol solution flow rate, ethanol concentration, and oxygen flow rate were evaluated by measuring the cell voltage as a function of current density for each set of conditions. The effect of the anode diffusion media was also studied. It was found that the cell performance was enhanced by increasing the cell temperature and the cathode backpressure. On the contrary, the cell performance was virtually independent of oxygen and fuel solution flow rates. Performance variations were encountered only at very low flow rates. The effect of the ethanol concentration on the performance was as expected, mass transport loses observed at low concentrations and kinetic loses at high ethanol concentration due to fuel crossover. The open circuit voltage appeared to be independent of most operating parameters and was only significantly affected by the ethanol concentration. It was also established that the anode diffusion media had an important effect on the cell performance. Journal of Power SourcesCrown

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Section: Effect Of Cathode Backpressure and Cell Temperaturementioning

confidence: 99%

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

Alzate

Fatih

Wang

2011

Journal of Power Sources

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“…Measurements of products from PEM-DEFCs have mainly been made by analyzing the anode exhaust solution only, 7,13,[23][24][25][26][27][28][29][30] which provides inaccurate results due to crossover of products to the cathode, and chemical oxidation of ethanol by the oxygen supplied to the cathode. 26,31 The latter problem can be avoided by using nitrogen and/or hydrogen at the cathode, 31 while the loss of products by crossover can be addressed by analysis of the cathode exhaust. 30,31 However, these methods have only been applied simultaneously in one study.…”

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“…26,31 The latter problem can be avoided by using nitrogen and/or hydrogen at the cathode, 31 while the loss of products by crossover can be addressed by analysis of the cathode exhaust. 30,31 However, these methods have only been applied simultaneously in one study. 32 In that work, the effects of crossover were quantified and various methods for determining n av at a Pt black anode were assessed.…”

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

Altarawneh

Pickup

2017

J. Electrochem. Soc.

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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...

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“…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.…”

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confidence: 97%

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confidence: 99%

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

Altarawneh

Pickup

2018

J. Electrochem. Soc.

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Measurement of the stoichiometry of ethanol oxidation is crucial in the development of direct ethanol fuel cell technology because it determines how efficiently the fuel is used. Here, it has been determined in fuel cell hardware at 80 • C for various anodes prepared with commercial Pt/C, PtRu/C and PtSn/C catalysts. Average numbers of electrons transferred per ethanol molecule (n av ) obtained from the flow rate dependence of the current (electrochemical method) are critically compared with values obtained from the amount of ethanol consumed and yields of acetaldehyde, acetic acid, and carbon dioxide (chemical analysis). In general, the electrochemical method provides accurate values of n av . Crossover of ethanol to the cathode causes a small (<10%) underestimation of n av , but this is generally less than the uncertainly of the measurement. However, when yields of carbon dioxide are high (>50%), the dependence of n av on the ethanol concentration can lead to its overestimation, and to impossibly high values. This can be diagnosed by comparing results obtained over different flow rate ranges, and the accuracy can be improved by use of a narrow range of flow rates. The electrochemical method can be routinely applied during measurement of a polarization curve, without any addition equipment, and is well suited for rapid assessment of the effects of operating conditions and changes over time. Ethanol is an attractive fuel for direct liquid fuel cells.1,2 However, the high theoretical efficiency of 98% for a direct ethanol fuel cell (DEFC) is based on complete oxidation of ethanol to CO 2 (Eq. 1), which generates 12 electrons (n = 12), while the main products from a DEFC are acetic acid (Eq. 2; n = 4) and acetaldehyde (Eq. 3; n = 2). 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,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. 11,12Determination of n av from the dependence of the current on the flow rate of the ethanol solution, in proton exchange membrane fuel cell (PEMFC) hardware, is particularly convenient. 11,13 In this method, the concentration of the...

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Effects of crossover on product yields measured for direct ethanol fuel cells

Cited by 58 publications

References 21 publications

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

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

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

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