(Invited) CO2 Electrolysis for Syngas Production

Abstract

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. 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. Sustainion™ 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. Figure 1 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. Figure 2 shows a steady state run at 600 mA/cm² using an optimal amount of Sustainion™-X6 in the catalyst layer. Notice that we can maintain 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. Figure 3 shows a steady state run at 200 mA/cm² using a Sustainion™-X1 membrane and an optimal amount of Sustainion™-X5 in the catalyst layer. Notice that we were able to maintain constant current for over 600 hours. Unfortunately, these runs ended abruptly when the power went off and then surged as it went on. But in runs without Sustainion™-X5 in the catalyst layer and no power surges, stable performance was observed for over 4000 hours. These results show that the ionomer and membrane can make a substantial difference to the performance of a CO2 electrolyzer. 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 or 3M. Figure 1