The Effect of Electrolyte on the Electrochemical Reduction of CO2 to CO

Carbon dioxide (CO₂) levels in the atmosphere have increased from 320 ppm in 1960 to more than 400 ppm in 2010, and continue to rise. These high CO₂ levels have been associated with mean temperature anomalies, erratic weather patterns and other adverse climate effects [1]. Multiple strategies have been proposed to lower CO₂ emissions with the hope to curb its negative impact. These include switching to renewable energy sources, increasing energy efficiency of consumer devices, vehicles and buildings as well as the capture and sequestration of CO₂ from fossil fuel based power plants [2]. Electrocatalytic or photocatalytic conversion of CO₂ to value added fuels and chemicals (e.g., carbon monoxide (CO), formic acid) could be another way of reducing CO₂ emissions while at the same time utilizing CO₂ in a useful way. Driving the electrocatalytic process with energy from renewable sources potentially provides a carbon neutral approach of producing carbon chemical feedstock. The process can further be used to accommodate the intermittent nature of many renewable energy sources.

While a lot of research in the past few decades has focused on the development of new catalysts, support materials, and electrolytes for electrochemical reduction of CO₂ to CO, desired performance levels for a viable process i.e., partial current densities for CO (j_CO) > 250 mA cm^-2 at energetic efficiencies (EE) > 60% are yet to be achieved [3]. Recently, Rosen et al. [4] demonstrated electrocatalytic conversion of CO₂ to CO using ionic liquid EMIM-BF₄ as the electrolyte at extremely low cell overpotential of 0.17 V. However, j_CO for the reaction was typically of the order of 1-5 mA cm^-2. In other work, we showed that using IrO₂ as the anode instead of Pt in combination with KOH as the electrolyte lowered the cell overpotential to 0.22 V. Much higher j_CO ≈ 250 mA cm^-2 was obtained at a cell potential of -3.00 V [5].

In this work we report the effect of different electrolytes on the electrocatalytic conversion of CO₂ to CO using Ag as the cathode and IrO₂ as the anode. All experiments were done in an electrochemical flow cell [6] at ambient conditions. Several electrolytes (KOH, KCl, KHCO₃, EMIM-Cl, Choline-Cl, and their combinations) were studied (Figure 1). The concentration of the electrolyte was found to play a major role in the process with j_CO increasing almost several folds upon going from 0.5 M to 3.0 M independent of the nature of the anion. j_CO as high as 440 mA cm^-2 at an EE of ≈ 42% and 230 mA cm^-2 at an EE ≈ 54% was observed when using 3.0 M KOH electrolyte. To the best of our knowledge, these are some of the highest j_CO levels reported to date in combination with energetic efficiencies exceeding 40%.

The effect of anions on electroreduction of CO₂ was also investigated. The onset cathode potential was found to shift in the order OH^- < HCO₃^- < Cl^- with the values being -0.13 V, -0.46 V, and -0.60 V vs. RHE, respectively. The Faradaic efficiency for CO (FE_CO) and j_CO was found to change in the reverse order. Furthermore, we also studied whether adding small amounts of EMIM-Cl or Choline-Cl to a KCl electrolyte had any effect on j_CO and FE_CO.

The experimental trends observed in this work can be explained by the interplay of a combination of factors, including pH, effect of conductivity and more importantly greater stabilization or destabilization of rate limiting CO₂^.- by anions and cations in the double layer. Electrochemical impedance spectroscopy was used to further support these arguments. Such an understanding can be crucial for designing more efficient electrocatalytic systems and can also open up avenues for utilizing low cost catalysts that have traditionally been not very active for CO₂ reduction.

Acknowledgment

We gratefully acknowledge financial support from AF STTR Phase II grant to Dioxide Materials, WPI-I²CNER and a 3M graduate fellowship to SV.

References

[1] D. H. Levinson, J. H. Lawrimore, Bull. Am. Met. Soc. 2008, 89.7, S1-S179.

[2] S. Pacala, R. Socolow, Science 2004, 305, 968-972.

[3] H. R. M. Jhong, S. Ma, P. J. A. Kenis, Curr. Op. Chem. Eng. 2013, 2.2, 191-199.

[4] B. A. Rosen, A. Salehi-Khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis, R. I. Masel, Science 2011, 334.6056, 643-644.

[5] S. Ma, R. Luo, S. Moniri, Y. Lan, P. J. A. Kenis, J. Electrochem. Soc. 2014, 161.10, F1124-F1131.

[6] D. T. Whipple, E. C. Finke, P. J. A. Kenis, Electrochem. Solid-State Lett. 2010, 13, B109-B111.