Generation of Pure Hydrogen by the Electrocatalytic Oxidation of Methane-Derived Liquid Fuels in a PEM Electrolysis Cell
Authors: Lamy Claude , Guenot Benoît , Cretin Marc 
Published: February 2020

The development of clean power sources and the reduction of the emission of greenhouse gases (e.g. CO2) have led to many investigations on Low Temperature Fuel Cells, such as the Proton Exchange Membrane Fuel Cell (PEMFC), fed either with pure hydrogen or with other fuels, particularly liquid fuels such as methanol or ethanol. Steam Methane Reforming (SMR) is the main process to produce hydrogen but it occurs at elevated temperatures (200 to 600°C) and produces a lot of CO2. In addition the reformate gas must be cleaned-up to reduce the content of CO to an acceptable amount (< 5 ppm), in order to avoid poisoning of the Pt-based catalysts of the PEMFC. Water electrolysis is a mature process, leading to very pure hydrogen. However, due to the high overvoltage encountered at the catalytic anode, where oxygen evolution does occur, the hydrogen production cost remains actually not competitive with the SMR. Therefore, instead of using water, other hydrogen containing compounds (particularly organic compounds) can be considered as hydrogen sources leading to pure hydrogen by their electro-oxidation at the anode of a Proton Exchange Membrane Electrolysis Cell (PEMEC) [1- 3]. This is a very challenging approach, since the theoretical cell voltage for their electrolysis (≈ a few tens mV) is much lower than the theoretical cell voltage of water electrolysis (1.23 V under standard conditions). Several organic compounds can be used as H2sources, such as carboxylic acids, alcohols, sugars, e.g. formic acid, methanol, ethanol, etc.
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
CH(OCH3)3 + 5 H2O → 4 CO2 + 20 H+ + 20 e-for TMM.
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:
CH3OCH3 + 3 H2O → 2 CO2 + 6 H2 for DME
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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