Electrochemical Reduction of CO2 to CO with High Selectivity Using an All Solid-State Electrolyzer Cell

The impact of climate change on our society is both real and immediate.  With rising sea levels, ocean acidification, loss of arctic ice cover, record high temperatures, and severe and lengthening droughts, ignoring CO₂ emissions is a risk that we simply cannot afford to take.  To combat climate change, we seek ways to both reduce and remediate anthropogenic greenhouse gas emissions.  For example, one method of reducing atmospheric CO₂ emissions is utilizing clean, renewable energy to electrochemically convert the effluent from carbon-intensive processes such as steel mills and cement factories into a more reactive form of carbon.  The resulting reactive form of carbon can then be used as a feedstock to existing industrial hydrocarbon syntheses, preventing the CO₂ from otherwise venting into the environment and providing a source of valuable raw materials.

Traditionally, the electrochemical reduction of CO₂ has required an overpotential on the order of 1.0 V to run efficiently owing to a very high-energy barrier intermediate.  In 2011, Dioxide Materials developed an ionic liquid-based helper catalyst that greatly lowered the overpotential required for electrochemical reduction of CO₂ to CO with high selectivity¹.  3M subsequently partnered with Dioxide Materials in a joint effort, largely funded by ARPA-e, to further develop this nascent technology into an industrial-scale process with the ultimate goal of finding less expensive routes of producing carbon-based fuels from CO₂.

This collaboration so far has yielded three different electrolyzer cell designs: an all liquid-based cell; an all solid-state (gas phase products and reactants) cell; and a hybrid liquid/solid design.  The solid state cell provides a number of advantages over the liquid cell, primarily owing to the reduced device complexity and thus lower overall operating costs; however, liquid-phase CO₂ reduction products (such as formic acid) are more difficult to isolate in such a system.  Each cell design has been tested in a number of configurations, including constant-current and constant-voltage, using a variety of ionic liquids and operating conditions, including liquid and gas flow rates, temperature, and electrolyte pH.  It was found that the product selectivity of the output gas stream depends upon all of these conditions.

Initially, our liquid-based cell gave us the best overall performance in terms of both selectivity and current density, achieving >99% CO selectivity (with H₂ being the dominant side product) at a current density of 50 mA/cm².  However, performance degradation was a serious drawback owing to a number of design challenges.  A follow-up study to the initial report on ionic liquid helper catalysts from Rosen et al. found that the ionic liquid anion (tetrafluoroborate in this case) played an important role in the overall product selectivity and faradaic conversion efficiency of the device².  While 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄) showed excellent results at suppressing water electrolysis, we found that BF₄^- readily hydrolyzed under standard operating conditions yielding HF acid which severely corroded the current collectors in the liquid cell design.  Utilizing an alternative ionic liquid such as EMIM-Cl exhibited similar results at water electrolysis suppression, giving high selectivity; however, the Cl^- anion was easily oxidized at the anode, yielding Cl₂ gas, resulting in a sharp pH gradient across the cell and a steady loss of selectivity over time.

More recently, we have made substantial progress on our solid-state design and have achieved >90% CO selectivity for more than 23 hours of continuous operation which represented a major breakthrough for the project (see Figure 1).  The realization of this design required the fabrication of new materials which will be discussed in greater detail in our presentation.  Our report will focus on our efforts at optimizing cell design and operating conditions in order to maximize CO production at a high selectivity and low cell voltage.  Included will be a discussion of studies into ionic liquid anion effect, pH dependence, and catholyte and anolyte flow conditions on device performance.

Acknowledgement.  The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-e), U.S. Department of Energy, under Award Number DE-AR000345.

References

(1)     Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. A.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I.  Science., 2011, 334, 643.

(2)     Rosen, B. A.; Zhu, W.; Kaul, G.; Salehi-Khojin, A.; Masel, R. I.  J. Electrochem. Soc. 2013, 160(2), H138.