274686 Oxygenate Production Using a Carbonate Anion Electrochemical Cell At Room Temperature
Oxygenate Production Using a Carbonate Anion Electrochemical Cell at Room Temperature
Neil Spinner and William E. Mustain
Department of Chemical, Materials and Biomolecular Engineering; University of Connecticut
Storrs, CT 06268
Typical heterogeneous methane (CH4) conversion processes are carried out at temperatures in excess of 700°C, needing high quality heat to drive the highly endothermic partial oxidation of methane to syngas [1-3]. The high energy, high temperature conditions can raise costs and impose material constraints that may potentially limit process feasibility and efficiency. Therefore, low temperature devices would be preferable if suitable catalysts can be found.
In developing a low temperature alternative, our group has recently focused on the development of new catalysts for room temperature electrochemical cells that operate on the carbonate anion cycle. Initial studies aimed to develop a carbonate-selective catalyst under fully humidified conditions to enable the direct electroreduction of O2 by CO2, Equation 1.
O2 + 2CO2 + 4e- ¦ 2CO3-2 (1)
This led to the synthesis and characterization of a novel pyrochlore-structured Ca2Ru2O7-y electrocatalyst [4,5]. Once carbonate is produced at the cathode of the room temperature carbonate cell (Figure 1), it is transported through an anion exchange membrane to the anode where it oxidizes an incoming fuel. In this study, our aim was to develop a new catalyst to partially oxidize methane to oxygenates at room temperature using carbonate anions as the oxygen donator.
Figure 1 – Room temperature carbonate electrochemical cell diagram with CH4 as fuel.
A nickel oxide-zirconia composite, shown in Figure 2, has been synthesized by a co-precipitation route for use as the anode electrocatalyst due to its ability to adsorb CO3-2 and provide electrocatalytically active sites. This anode is unique not only due to its bifunctional catalytic capabilities, but also in its use of a non-conductor (zirconia) to promote an electrochemical reaction.
Figure 2 – Scanning Electron Microscopy (SEM) micrographs for (A) pure nickel oxide; and (B) nickel oxide-zirconia composite anode electrocatalyst.
In this cell, depending on the cell construction, various hydrocarbon products are formed, including syngas and formaldehyde:
CO3-2 + CH4 ¦ CO + CO2 + 2H2 + 2e- (2)
2CO3-2 + CH4 ¦ HCHO + CO2 + H2O + 4e- (3)
Analysis of the anode effluent by mass spectrometry for a typical cell geometry is shown in Figure 3, and the peak locations indicate formaldehyde as the primary product formed, though other geometries are CO-selective.
Electrochemical activity was analyzed in alkaline aqueous solutions through Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) using thin-film disk-type electrodes, and products were identified from fully-constructed electrochemical cells using Mass Spectrometry (MS), Gas Chromatography (GC), and Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR). Insight into the potential pathways and mechanisms of various CH4 activation reactions will be discussed as well.
Figure 3 – Mass spectrometry histogram for anode effluent collected from room temperature carbonate electrochemical cell operating with methane fuel.
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