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

Abstract

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...

Pt/C and Pt/SnOx/C Catalysts for Ethanol Electrooxidation: Rotating Disk Electrode Study

“…Moreover, according to Camara and Iwasita, CO 2 evolution is more pronounced at low EtOH concentrations (<0.1 M), with negligible acetaldehyde [8]. Recently, an effective PEM-based cell approach was established [55][56][57], which enables one to accurately determine the n av and stoichiometry of the EOR using the PEM electrolysis cell operated in crossover mode. The thick-film RDE technique that we used, and describe here, could nonetheless be used as an approach for quick catalyst screening.…”

A mathematical model for direct ethanol fuel cells based on detailed ethanol electro-oxidation kinetics

“…where the variation of the cathode overpotential with C O 2 ,cc and i c is given analytically by Eq. (9), while the explicit dependencies of the current densities i and i c as a function of C E,ac , C A,ac and η a result from the solution of the 1D anode model (see Appendix for details), which provides the output current density i and enables the computation of i c through Eqs (7), (18) and (19).…”

“…expressed here in terms of the output and parasitic current densities i, i E,p and i A,p with use made of Eqs. (13), (18), (23) and (24).…”

“…The most thorough investigation of the faradaic efficiency of ethanol electro-oxidation has been carried out by Pickup et al [15,16]. In a long series of papers, Pickup's group has developed or applied a large variety of experimental techniques (operating DEFCs in crossover mode [17], using the flow rate dependence of the current to estimate the faradaic efficiency of ethanol oxidation [16,18], pulsing the potential or current to increase CO 2 yields [19,20,21], applying different methodologies for the online analysis of DEFC products [22,23], etc.) combined with simple but insightful experiment-based models [16,24], to analyze systematically both PEM electrolysis cells and DEFCs.…”

Overview on the Vital Step toward Addressing Platinum Catalyst Poisoning Mechanisms in Acid Media of Direct Ethanol Fuel Cells (DEFCs)

“…In this regard, computational studies − and various experimental techniques including chromatography, ,− differential electrochemical mass spectrometry (DEMS) − in situ Fourier transform infrared spectroscopy (FTIR), ,,− and nuclear magnetic resonance (NMR) − have been successfully employed to give insights regarding the mechanism of the EOR occurring at the developed anode catalysts. Briefly, DEMS has proven to be a useful technique to investigate and quantify the volatile and gaseous species that are generated during the electrochemical reaction.…”