(685d) Bench-Scale Electrochemical Production of Synthesis Gas

Bench-Scale Electrochemical Production of
Synthesis Gas

Eric
J. Dujek, Tedd E. Lister,
and Michael E. McIlwain

Idaho
National Laboratory

Syn-gas
is a very useful mixture of H₂ and CO that can be utilized to
synthesize a great variety of commodity chemicals and transportation fuels
using established methods. Conventional methods create syn-gas
using fossil resources which releases carbon into the atmosphere.
Electrochemistry offers a clean method of producing this gas mixture. Two major
technical issues exist for efficient electrochemical reduction of CO₂:
1) mass transport limitations due to the limited aqueous solubility of CO₂
and 2) kinetic limitations leading to loss of energy efficiency. The first
limitation can be dealt with by using gas diffusion electrodes (GDE) and
pressurized cells. The second limitation results in reduced efficiency and will
require additional catalyst development to improve. However, there may be situations
were low value electricity or grid isolated energy sources (wind and solar)
could overcome efficiency losses. Thus this process could be configured to act
as a load leveling device for intermittent electrical supplies and converting
this energy to a useful product. This abstract describes a project which has
used technical data from previous work in both CO₂ and H₂O
electrochemical reduction to build electrochemical systems to produce syn-gas mixtures. This project's goal is to collect data to
enable pilot scale design and production.

To
increase the current density for CO₂ reduction, mass transport must
be increased beyond that possible on conventional flat electrodes. Previous
reports have shown increased partial current densities for CO₂
reduction at GDEs.[1]
The systems described here employ a robust, commercial GDE incorporating a
porous Ag catalyst structure called Silflon. The Ag
catalyst was chosen to produce CO as the primary product of CO₂
reduction.[2]
Another approach to increasing current density is to use a cell pressurized
with CO₂. A cell equipped with a Ag GDE and
enclosed in an autoclave at 30 atm of CO₂
demonstrated over 3 A/cm² partial current density for CO formation.[3]  With this previous work as a guide, an
alternative pressurized electrolysis system has been constructed which operates
at elevated CO₂ pressure. Both the ambient pressure and elevated
pressure systems utilize filter-press cell designs to minimize resistive losses
and provide data for scale-up. Dimensionally stable anodes were used to evolve
O₂ from a caustic solution. The cathode solution chemistry is
largely determined by carbonate acid-base equilibria.
While protons are consumed by the cathodic reactions,
CO₂ reacts with hydroxide to produce carbonate salts. A variety of
solution chemistries and both anion exchange membranes (AEM) and cation exchange membranes (CEM) have been studied.

Using
the ambient pressure system a number of process variables were investigated.[4]
Electrolysis cells such as those used in alkaline water electrolysis operate at
elevated temperature where resistive and kinetic losses heat the system. The
increased temperature reduced the overall cell voltage and cathode voltage,
particularly at higher current densities. The Faradaic
efficiency (FE) for CO decreases slightly with temperature. Increasing the current
density has a more pronounced effect on the FE for CO, where the H₂:CO ratio increases. A significant benefit of the reduced
cell voltage is the significant improvement in electrical efficiency with
increasing temperature. A well designed commercial scale cell would be able to
utilize the electrical inefficiency as a source of heat.

Another
factor controlling the H₂:CO ratio is the
CO₂ flow rate into the GDE. As expected, the ratio is inversely
proportional to the CO₂ flow rate. By controlling the flow rate it
was possible to achieve specific H₂:CO
ratios with CO accounting for 25-90% of the product and H₂
accounting for 10-75%. This provides significant product selectivity control needed
to provide specific syn-gas ratios for downstream
conversion to hydrocarbon products.

Reproducibility
of cell performance was also evaluated due to the importance of reproducibility
for syn-gas production. Using two different GDEs the
experimental reproducibility was evaluated using a minimum of four data points
per current density evaluated. Each run involved stepping the current density
at 70˚C using 0.8 M K₂SO₄ as the catholyte
and a CO₂ flow rate of 30 mL min^-1.
The data show that the system was reproducible both in terms of the potentials
necessary for CO₂ reduction, but also in its ability to produce a
specified syn-gas composition.

System
stability was demonstrated by operating the cell for many hours at various
operating conditions. After an initial stabilization period of 40 min,
long-term cell performance was stable. The present design has shown bench scale
reduction of CO₂ can be performed at current densities which are
near those currently employed in industrial alkaline electrolysis cells.

The
influence of S poisoning on Ag electrodes for the production of synthesis gas (syn-gas) was evaluated.[5]
After exposure to Na₂S at open circuit potential (OCP) the overpotential for H₂ evolution decreased
resulting in a significant decrease in the Faradaic
efficiency for CO. It was found that poisoning was mostly reversed by
performing electrolysis at 20˚C in S free electrolyte. However, at
70˚C this recovery was not possible. Measurements on planar disk
electrodes showed distinct stripping waves for S adsorbed at OCP. The stripping
waves were influenced by both temperature and the presence of CO₂.
These measurements show the potential dependant nature of electrode recovery
where at elevated temperature adequate polarization to strip S was prevented by
H₂ evolution. Poisoning during operation leads to a temporary
decrease in CO produced which can be mostly recovered by exchanging the catholyte. SEM analysis of S-exposed GDEs demonstrated a
blocking of the pore-structure at the GDE surface. Preliminary experiments with
economical grades of CO₂ showed only minor poisoning due to
S-containing species leading to only marginal changes in syn-gas
composition.

The impact of membrane type and electrolyte
composition was also investigated.[6]The electrolyte lifetime was found to be limited by hydroxide composition
in the anolyte, where a reduction in pH results in a
significant increase in cell voltage. By changing from a nafion-based
CEM to an AEM extended the cell operational time at low cell voltages (E_cell) without impacting product composition.
The use of KOH as the catholyte decreased the E_celland resulted in a minimum
electrolyte cost reduction of 39%.

In
addition to the ambient pressure measurements described above, work using
pressurized CO₂ has been performed.[7]
The pressurized system displays reproducible behavior at both room and elevated
temperatures and the quantity of CO which can be generated is 5 times that
observed using similar electrodes at ambient pressure. Above 15 atm it is possible to generate CO at 80% FE at 225 mA cm^-2. The FE for CO was 92% at 24.7 atm and 350 mA/cm².
It was found the pressurized cell operates at lower cell voltage than obtained
using the same electrode materials at ambient pressure. A cell voltage below 3
V was obtained at 225 mA/cm²,
which equates to an electrical efficiency of 50%. Work is continuing to
develop this system to couple the electrolysis products to catalytic systems
for conversion to hydrocarbon products.