(492c) Economic Production of Syngas from CO2 and H2O: New Routes to Synthetic Fuels

Economic production of syngas from CO₂ and H₂O could produce new routes to synthetic fuels. In this paper we will describe recent work on the electrolysis of CO₂ in a 5 cm² alkaline electrolyzer using standard Fuel Cell Technologies fuel cell hardware. Previous work had demonstrated CO₂ conversion with steady state currents of 50 mA/cm² or less at 3 V. In this paper we find that by optimizing the composition of the membrane and ionomer we can raise the current to 280 mA/cm² at 3V and 600 mA/cm² at 3.25-3.35 V.

Table 1 shows how the performance of the electrolyzer varies with membrane composition for several different membranes using bare silver cathode catalysts, ruthenium anode catalysts and a variety of membranes. In all cases the measurements were done by sandwiching the membrane between two pieces of carbon cloth that were coated with membrane, mounting the resultant MEA in 5 cm² fuel cell hardware, supplying humidified CO₂ to the cathode, applying 3 V, waiting 20 minutes to 1 hr for the current to stabilize and recording the current.

Membrane	Faradaic Efficiency	Current at 3V (mA/cm2)	Current for carbon containing products (mA/cm2)
Nafion 117	0%	72	0
Neosepta	34%	24	8
Acid doped CMI-7000	0.02%	35	0.007
Alkali doped AMI-7001	25%	10	2.5
SPEEK	2.5%	5	0.13
Alkali doped PVA	52%	15	7.5
Alkali doped PEI/PVA	16%	10	1.6
H3PO4 doped PBI	14.7%	8	1.2
Sustainionâ?¢ X1	95%	80	75
Sustainionâ?¢ X2	93%	80	72
Sustainionâ?¢ X3	83%	25	20.8

Notice that the faradaic efficiency for the conversion of CO₂ to CO varies from 0 to 95%. While the current varies from 5 to 80 mA/cm². Sustainionâ?¢ membranes give higher currents and faradaic efficiency than the others. These results demonstrate that the membrane composition makes a significant difference to the electrolyzer performance.

We have also determined shows how the polarization curve of the cell varies as we add different amounts of Sustainion�-X5 to the cathode layer in a cell with a Sustainion�-X4 membrane. Notice that one can obtain currents of 1.4 amps (280 mA/cm²) at 3 volts with a Sustainion�-X4 membrane and and Sustainion�-X5 ionomer.

Steady state runs were done at 200 and 600 mA/cm². The cell is able to sustain 200 mA/cm² a 2.9 V and 600 mA/cm2 at between 3.25 and 3.35 V. By comparison, the closest competitor needed almost 7 V to obtain the same current.

These results demonstrate that one can use electrolyzers with Sustanionâ?¢ membranes to convert CO to CO₂ at industrially important rates

Acknowledgement

Parts of this work were supported by ARPA-E under contract DE-AR0000345 and by 3M. The opinion are those of the authors and may not reflect the opinions of ARPA-E.