Other organic compounds can be interesting for producing very pure hydrogen by their electrochemical decomposition, particularly those derived from methane (natural gas, shale gases, etc.) or from methanol (which is mainly produced by the conversion of “Syngas”, a mixture of CO and H2), such as dimethyl ether DME (CH3OCH3), dimethoxymethane DMM [CH2(OCH3)2] and trimethoxymethane TMM [CH(OCH3)3]. Like methanol these ethers and poly-ethers can be easily oxidized at the Pt-Ru anode of a DMFC hardware since they do not contain a C-C bond (conversely to ethanol). Moreover they have an energy density (≈ 7 to 9 kWh kg-1) higher than that of methanol (≈ 6 kWh kg-1) so that they have been considered as alternative fuels for a Direct Oxidation Fuel Cell [4, 5].
Their complete oxidation to carbon dioxide in an acid electrolyte (like a PEM) involves 12 to 20 electrons, respectively, as follows:
CH3OCH3 + 3 H2O → 2 CO2 + 12 H+ + 12 e-for DME
CH2(OCH3)2 + 4 H2O → 3 CO2 + 16 H+ + 16 e-for DMM
Then the protons produced at the anode cross-over the protonic membrane and reach the cathodic compartment of the PEMEC, where they are reduced to hydrogen, 2 H+ + 2 e- → H2, leading to pure H2 by the electrochemical decomposition of these organic compounds, as follows:
CH2(OCH3)2 + 4 H2O → 3 CO2 + 8 H2 for DMM
CH(OCH3)3 + 5 H2O → 4 CO2 + 10 H2 for TMM.
In this communication the electro-reactivity of several methane-derived liquid fuels, such as poly-ethers, including HCHO and CH3OH (which can result from the hydrolysis of ethers in acidic media), was compared by cyclic voltammetry. Then the electrochemical decomposition of methanol and DMM for the production of clean hydrogen was investigated in a Direct Methanol Fuel Cell (DMFC) hardware of 5 cm2 geometric surface area (with a Pt-Ru/C anode and a Pt/C cathode) working as a PEMEC. The anodic compartment was fed with an acidic solution (0.5 M H2SO4) containing the organic compound (with concentrations from 0.1 M to 10 M) whereas the cathodic compartment was fed with a pure 0.5 M H2SO4 solution. The electrolysis cell, thermostated at several controlled temperatures (25 to 85°C), was polarized at a constant current density j (from 1 to 100 mA cm-2). Both the cell voltage of the electrolysis cell and the volume of hydrogen produced were recorded as a function of time t and current density j.
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