(553f) 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 Ca₂Ru₂O_7-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.

References:

1.      
F. Lopez et al. Powder Technol., 219 (2012) 186.

2.      
A. Pushkarev, A. Zhu, X. Li, R. Sazonov. High Energy Chem., 43
(2009) 156.

3.      
M. Karakaya, S. Keskin, A. Avci. Appl. Catal. A: Gen, 411-412
(2012) 114.

4.      
J. Vega, S. Shrestha, M. Ignatowich, W. Mustain. J. Electrochem. Soc.,
159 (2012) B12.

5.      
J. Vega, N. Spinner, M. Catanese, W. Mustain. J. Electrochem. Soc.,
159 (2012) B19.