New anode for direct ethanol fuel cells enables peak power and current densities approaching H2 PEM fuel cells
A team of researchers in Italy has developed a new palladium-doped anode for direct alcohol fuel cells that produces peak power and current densities (using ethanol at 80 °C) approaching the output of hydrogen-fed proton exchange membrane fuel cells (PEMFCs). A paper on their work is published in the RSC journal ChemSusChem.
Direct alcohol fuel cells (DAFCs), which belong to the family of alkaline fuel cells, are electrochemical devices that continuously convert the chemical energy of an alcohol fuel to electricity. Ethanol is becoming a desirable target fuel for use in DAFCs (i.e., a DEFC) because it offers higher energy density compared to methanol; less crossover rate (from the anode to cathode); and can be produced from agriculture and biomass products. In a 2006 paper (Mann et al.), researchers at Princeton observed that:
The direct 12-electron oxidation of ethanol to carbon dioxide and water in a fuel cell reactor offers a potentially attractive energy resource. In contrast to the much studied hydrogen−oxygen fuel cell, ethanol provides a volumetric energy density that approaches that of gasoline (21 MJ L-1 for ethanol vs 31 MJ L-1 for gasoline). As a liquid fuel, ethanol also avoids issues of storage associated with proposed hydrogen systems. Assuming that ethanol is bioderived, this materials is considered to be carbon neutral. The direct ethanol fuel cell’s (DEFC) primary disadvantage is the lack of a catalyst that can initiate complete oxidation at a high rate. In the absence of an electrocatalytic system that can efficiently deliver 12 electrons per ethanol molecule, the optimistic picture suggested above vanishes.
The electrochemical oxidation of ethanol is a difficult task because of the substantial increase in the number of reaction intermediates associated with this 12-electron process. More troublesome is the presence of the C−C bond, which is between two atoms with little electron affinity or ionization energy, thus making it difficult to access electrochemically.—Mann et al.
|The complete oxidation of ethanol molecule involves the release of 12 electrons and the cleavage of the C-C bond, which is between two atoms with little electron affinity or ionization energy.|
|Anodic ethanol oxidation: C2H5OH + 3H2O → 2CO2 + 12H+ + 12 e-|
|Oxygen reduction reaction: 3O2 + 12H+ + 12e- → 6H2O|
|Full reaction of DEFC: C2H5OH + 3O2 → 2CO2 + 6H2O|
Key to the success of the DEFC is the catalyst. Many catalysts have been developed and demonstrated electrochemically to oxidize small alcohols, but with varying degrees of oxidation.
To avoid the drawbacks of carbon-supported nanoparticle (NP) electrocatalysts, the team ICCOM CNR (Institute of Organometallic Chemistry, National Research Council) in Italy prepared anodes consisting of palladium (Pd) NPs supported on 3 D TiO2 nanotube arrays.
A 2 μm thick layer of TiO2 nanotube arrays was prepared on the surface of the Ti fibers of a nonwoven web electrode. After it was doped with Pd nanoparticles (1.5 mg Pd cm−2), this anode was employed in a direct alcohol fuel cell. Peak power densities of 210, 170, and 160 mW cm−2 at 80 °C were produced if the cell was fed with 10 wt % aqueous solutions of ethanol, ethylene glycol, and glycerol, respectively, in 2 m aqueous KOH.
The Pd loading of the anode was increased to 6 mg cm−2 by combining four single electrodes to produce a maximum peak power density with ethanol at 80 °C of 335 mW cm−2. Such high power densities result from a combination of the open 3 D structure of the anode electrode and the high electrochemically active surface area of the Pd catalyst, which promote very fast kinetics for alcohol electro-oxidation.—Chen et al.
Chen, Y., Bellini, M., Bevilacqua, M., Fornasiero, P., Lavacchi, A., Miller, H. A., Wang, L. and Vizza, F. (2014), “Direct Alcohol Fuel Cells: Toward the Power Densities of Hydrogen-Fed Proton Exchange Membrane Fuel Cells,” ChemSusChem doi: 10.1002/cssc.201402999
A. M. Sheikh, Khaled Ebn-Alwaled Abd-Alftah, C. F. Malfatti (2014) “On reviewing the catalyst materials for direct alcohol fuel cells (DAFCs),” Journal of Multidisciplinary Engineering Science and Technology Vol. 1 Issue 3
Jonathan Mann, Nan Yao, and, and Andrew B. Bocarsly (2006) “Characterization and Analysis of New Catalysts for a Direct Ethanol Fuel Cell” Langmuir 22 (25), 10432-10436 doi: 10.1021/la061200c